I guess there is still lots of noob questions in me despite more than 2 years of messing around with Gadgeteer 
I want to build my own module and need at least 3 interrupt capable pins, according the the Gadgeteer port specs there is no sockets with that capability. Can that really be true?
Do I need all sorts of OR-circuitry and then poll which port did the triggering?
there is no guarantee that you can do that on a single socket. You can achieve it one of two ways - use multiple sockets, or add your own interrupt capability via circuitry, or don’t label it as a Gadgeteer socket and test the pins and fail if you can’t get 3x interrupt pins as you need. Yes, I added the third option in there but only said two ways, because ignoring it won’t be easy and is going against “the Gadgeteer way”…
If this is just a module for you, that you’ll hardly ever move to different sockets, then I’d actually use option 3 and hardcode the socket you use to one you know will work.
tell us more about what you want to do, what the three IRQ lines are from etc and there might be other options