Going to need a few triggers to do it, I can think of a few ways not sure which is the simplest.
You could show a layer that is shown each time you press a button and then do a count of how many are selected and if equal to 2 disable the button that was not equal to selected.
Or you could add triggers if
A+B = selected disable C
A+C = selected disable B
B + C = selected disable A
You then need triggers to change back to normal and the layer option is probably better here as you can use the count to change any disabled back to normal
