Stacking UDN2981 ICs for more Amps
The output stage of the UDN2981 is bipolar, so you must always fear the negative temperature coefficient – if one of the chips heats up more than the other, it will hog more current and this results in a quick thermal runaway of the stack.

So, two of them tightly stacked might be okay. Three – not, because the one in the middle inevitably gets much warmer than the outer ones.

Putting balancing resistors between Vdd and Vs is possible (or on each output) but still seems a bad idea.