birch countertop: how many coats of polyurethane?

I put up a sideboard work surface in my office and applied three coats of water-based satin polyurethane (light sanding between coats). Is three coats enough? Some computer equipment will be sitting on it, as well as papers, and the occasional coffee mug.
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that'll be fine. >I put up a sideboard work surface in my office and applied three coats

Someday, it'll all be over....
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