Can you use the pass for Colosseum/Palatine, Forum on one day, then not use it the next for any sites (we're going to St. Peters), the the Bourgese gallery for the 3rd day?
If I've understood your question correctly, yes. You might not be able to space it a month apart, but you do not have to use the pass on consecutive days.
The Roma Pass consists of TWO passes: one is for the attractions and the other is for transportation. Once you activate the transportation pass (which is separate from the other), then you have three consecutive days to use the transportation pass. The other is not affected.
You have two consecutive days plus the remainder of the first day till 12am, in which the card is activated....