one content place holder hiding behind another content place holder.

Hi,
 in all my pages there are three content place holders / colums ... say leftcol, centercol and rightcol.
when my browser window is full size, all three columns appear clean and neat.
But when I minimize the size of browser by dragging from right to left, I see that rightcol gradually hides behind centercol instead of remaining static.
how can i resolve this defect?
Thanks,
NetQuestions
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