1.两侧固定，中间自适应
.box{
width: 100%;
display: flex;
height: 300px;
}
.left{
width: 100px;
height: 200px;
background-color:red;
}
.right{
width: 100px;
height: 200px;
background-color:red;
}
.center{
flex: 1;
height: 300px;
background-color: lime;
}
<body>
<div class="box">
<div class="left"></div>
<div class="center"></div>
<div class="right"></div>
</div>
2.左侧固定，右侧自适应
.box{
width: 100%;
display: flex;
height: 300px;
}
.left{
width: 100px;
height: 200px;
background-color:red;
}
.right{
flex: 1;
height: 300px;
background-color:lime;
}
<div class="box">
<div class="left"></div>
<div class="right"></div>
</div>