I am building a house that has an interior a/c unit (electric heat pump) on each of the 3 floors of the house. In a previous house I had some wood floors ruined because the drain in the a/c unit plugged and caused water to leak from the unit. I wanted to know if it is possible to put an overflow pan under a/c units (like one used for washers) so that I do not have a repeat of the leak problem. Thanks.

It is common practice. Drains plug up all too often. Big problem is that they are small and have too many ells. To reduce the hassle of backed up drains, insist on 3/4'' pipe and as few bends as possible. An overflow pan only delays problems unless it is monitored, or has another, separate drain.