Grimrock

I found the 3 buttons and they allow me to get out again after triggering the poison trap, but I cant seem to figure out how to open the door in

Same buttons, just a different combination. I don't recall the combination, but the last button I clicked was the one at the end, on the right... and I'm not sure whether I came back to press the first one, or I pressed it before stepping into the pressure plate. I think I didn't go back, though.

Turns out it was left, entrance, right. So you have to quickly get back and forth before you hit the switch in the middle and release a second wave of poison