CSS中心元素在3列布局

So I have a 3-column layout on my webpage, but I can't get the things in the middle column to be centered. The columns on the left and right are of fixed width, so I created a container for the middle column and set its borders to equal the size of the left and right columns. Then, I used the margin:auto property on the div tag inside the middle container that has everything that I want in the middle column. If possible, I want this to work on all browser window sizes. This is my CSS:So I have a 3-column layout on my webpage, but