Select the last 3 child elements

Question

I know you can select the last child with :last-child or a certain child with :nth-child (if you know their position/number).

What I need is to

Answer 1

The accepted answer has the correct formula, but the explanation is wrong.

So the correct CSS is (same as currently accepted answer):

#something a:nth-last-child(-n+3) {
    /*declarations*/
}

But here is the correct explanation of the math:

f(n) = -n+3
f(0) = -0+3 = 3 <- 3rd row from the bottom
f(1) = -1+3 = 2 <- 2nd row from the bottom
f(2) = -2+3 = 1 <- 1st row from the bottom
f(3) = -3+3 = 0 <- nothing
f(3) = -4+3 = -1 <- nothing
etc...