Red claw crab
 

So I just had to move the guys from my 15 gallon to my 75 due to the malfunctioning heater (went to do a water change and water felt hot to my hand, it was at 92...) and I'm thinking about using it for crabs.

From what I've read so far feeding shouldn't be a problem at all, set up and closing any hole should be easy, I just don't know if I should only get 2 or get 3. Two seems like a better choice given the small space but I came by something saying pushing the space limit will make them more active.

Any tips on these things?

they need to go above the waterline for air. So you will have to have space for that.