Gutter between divs

Answer 1

you may select the second div and reset width and margins:

.col-1-3:nth-child(2) {
  margin:0 1em;
    width:calc(100% / (3/1) - 2em);
}

.container {
  font-size: 0;
}
[class|="col"] {
  display: inline-block;
  vertical-align: top;
  position: relative;
  font-size: 20px;
}
.col-1-3 {
  width: calc(100% / (3/1));
}
/* here update to overwrite width and add margins rules */
.col-1-3:nth-child(2) {
  margin: 0 1em;/* whatever , double it before taking it off from width */
  width: calc(100% / (3/1) - 2em);
}
/* end update */
.col-2-3 {
  width: calc(100%/(3/2));
}
.col-1 {
  width: 100%;
}
.bg-blue {
  background-color: #42a5f5;
  color: #ffffff;
}
.bg-green {
  background-color: #66bb6a;
  color: #ffffff;
}

<div class="container">
  <div class="col-1-3 bg-blue">blue left</div>
  <div class="col-1-3" style="font-size:0px;">
    <div class="col-1-3 bg-green">green 1</div>
    <div class="col-1-3 bg-green">green 2</div>
    <div class="col-1-3 bg-green">green 3</div>
  </div>
  <div class="col-1-3 bg-blue">blue right</div>
</div>

You may also want to take a look at display:flex :

.container{
  display:flex;
}
[class|="col"] {
    flex:1;/* flex children, spray them evenly */
    font-size:20px;
}
.container .container .col-1-3:nth-child(2) {/* select second containere on second level */
  margin:0 1em;
}
.bg-blue{
	background-color:#42a5f5;
	color:#ffffff;
}
.bg-green{
	background-color:#66bb6a;
  color:#ffffff;
}

<div class="container">
  <div class="col-1-3 bg-blue">blue left</div>
	<div class="col-1-3 container"><!-- here make it a flex container too -->
    
    <div class="col-1-3 bg-green">green 1</div>
		<div class="col-1-3 bg-green">green 2</div>
		<div class="col-1-3 bg-green">green 3</div>
      
  </div>
	<div class="col-1-3 bg-blue">blue right</div>
</div>

Codepen to play with https://codepen.io/anon/pen/xEqQVg

edit

A third option, float + display:table, bg drawn via faux column . This means to put right and left column in front of the middle container.

.container{
	background-color:#42a5f5;
  margin:1em 0;
  text-align:center;
  font-size:20px;
  line-height:30px;
  text-shadow:0 0 1px gray;
}
.bg-blue {
  width:33%;/* whatever how many of those ? */
	background-color:#42a5f5;
	color:#ffffff;
}
.float-l {
  float:left;
  margin-right:-15px;/* cause of border-spacing */
}
.float-r {
  float:right;
  margin-left:-15px;/* cause of border-spacing */
}
.child-container {
  display:table;
  table-layout:fixed;/* suppose to spray cells evenly where no width is set to cells */
  background:linear-gradient(to left,#42a5f5 15px, transparent 15px),linear-gradient(to right,#42a5f5 15px, transparent 15px) white;/* reproduce blue bg part on white for both sides */
  width:calc(34% + 30px);
  margin:auto;
  border-spacing:15px 0;
}
.bg-green{
  display:table-cell;
	background-color:#66bb6a;
  color:#ffffff;
}

<div class="container">
  <div class="float-l bg-blue">blue left</div>
	<div class="float-r  bg-blue">blue right</div>
	<div class="child-container">    
    <div class=" bg-green">green 1</div>   
  </div>
</div>
<div class="container">
  <div class="float-l bg-blue">blue left</div>
	<div class="float-r  bg-blue">blue right</div>
	<div class="child-container">    
    <div class=" bg-green">green 1</div>
		<div class=" bg-green">green 2</div>   
  </div>
</div>
<div class="container">
  <div class="float-l bg-blue">blue left</div>
	<div class="float-r  bg-blue">blue right</div>
	<div class="child-container">    
    <div class=" bg-green">green 1</div>
		<div class=" bg-green">green 2</div>
		<div class=" bg-green">green 3</div>   
  </div>
</div>
<div class="container">
  <div class="float-l bg-blue">blue left</div>
	<div class="float-r  bg-blue">blue right</div>
	<div class="child-container">    
    <div class=" bg-green">green 1</div>
		<div class=" bg-green">green 2</div>
		<div class=" bg-green">green 3</div>    
    <div class=" bg-green">green 4</div>    
  </div>
</div>

codepen https://codepen.io/anon/pen/qaAvjX

Answer 2

I think adding this to your css, would do the job for you.

.col-1-3.bg-green{
    width:calc(100%/(3/1) - (3-1)*2px);
  margin:0px 2px;
}

Answer 3

.container{
 display:flex;
  
  
  
}
.container > div{
  flex-grow:1
}
[class|="col"] {
    display:inline-block;
    vertical-align: top;
    position:relative;
    font-size:20px;

}
.col-1-3{
	
}
.col-2-3{
	
}
.col-1{
	
}
.bg-blue{
	background-color:#42a5f5;
	color:#ffffff;
}
.bg-green{
	background-color:#66bb6a;
  color:#ffffff;
}

.bg-green+ .bg-green{ margin-left:20px}

<div class="container">
  <div class="bg-blue">blue left</div>


  <div class=" bg-green">green 1</div>
  <div class=" bg-green">green 2</div>
  <div class=" bg-green">green 3</div>
  


  <div class=" bg-blue">blue right</div>
</div>

Something like this? This is using flexbox layout. If you want more browser compatibility then I think a good option would be use any css framework.

Answer 4

You can add this to your CSS and it should work (tested):

.bg-green {
  width: calc((100% - ((3 - 1)*2%))/(3/1));
  margin-right: 2%;
}

.bg-green:last-child{
  margin-right: 0;
}

Edited snippet:

.container{
  font-size: 0;
}
[class|="col"] {
    display:inline-block;
    vertical-align: top;
    position:relative;
    font-size:20px;

}
.col-1-3{
    width:calc(100%/(3/1));
}
.col-2-3{
    width:calc(100%/(3/2));
}
.col-1{
    width:100%;
}
.bg-blue{
    background-color:#42a5f5;
    color:#ffffff;
}
.bg-green{
    background-color:#66bb6a;
  color:#ffffff;
}

.bg-green {
  width: calc((100% - ((3 - 1)*2%))/(3/1));
  margin-right: 2%;
}

.bg-green:last-child{
  margin-right: 0;
}

<div class="container">
  <div class="col-1-3 bg-blue">blue left</div>
    <div class="col-1-3" style="font-size:0px;">

    <div class="col-1-3 bg-green">green 1</div>
        <div class="col-1-3 bg-green">green 2</div>
        <div class="col-1-3 bg-green">green 3</div>

  </div>
    <div class="col-1-3 bg-blue">blue right</div>
</div>

Just replace "3" with whatever numbers of columns you have and change the margin-right value as you like.

Version with negative margin

Change markup a little bit (similar structure to Bootstrap):

<div class="col-1-3">
  <div class="bg-green">green 1</div>
</div>
<div class="col-1-3">
  <div class="bg-green">green 2</div>
</div>
<div class="col-1-3">
  <div class="bg-green">green 3</div>
</div>

And following part in CSS

.children-has-gutters > div {
    padding-left: 15px;
    padding-right: 15px;
    box-sizing: border-box;
}

.bg-blue {
    background-color:#42a5f5;
    color:#ffffff;
    width:calc((100%/(3/1)) + 15px);
}

.container{
  font-size: 0;
}
[class|="col"] {
    display:inline-block;
    vertical-align: top;
    position:relative;
    font-size:20px;

}
.col-1-3{
	width:calc(100%/(3/1));
}
.col-2-3{
	width:calc(100%/(3/2));
}
.col-1{
	width:100%;
}

.children-has-gutters{
  margin-left:-15px;
  margin-right:-15px;
  width: calc((100% / (3/1)) + 30px);
}
.children-has-gutters > div{
  padding-left:15px;
  padding-right:15px;
  box-sizing: border-box;
}


.bg-blue{
	background-color:#42a5f5;
	color:#ffffff;
}
.bg-green{
	background-color:#66bb6a;
    color:#ffffff;
}

<div class="container">
  <div class="col-1-3 bg-blue">blue left</div>
	<div class="col-1-3 children-has-gutters" style="font-size:0px;">
    
    <div class="col-1-3">
      <div class="bg-green">green 1</div>
    </div>
    <div class="col-1-3">
      <div class="bg-green">green 2</div>
    </div>
    <div class="col-1-3">
      <div class="bg-green">green 3</div>
    </div>
      
  </div>
	<div class="col-1-3 bg-blue">blue right</div>
</div>

Note that you had to add 15px to each blue column, to avoid issue with container width. You can play around and check what happens without it - width of elements in container would be 100% - 30px (negative margin).