I am sure you can feel it from the inside, I only know this because I'd been with a chick and used a toy on her at the same time and if I'm in her ass and the vibe is in her pussy I can feel the vibe going in and out.
Personally, the MMF threesome is just not for me. I don't know if I could stay hard with another erect dude in the room. Under the right circumstances I suppose I would try it. But fuck, a dudes balls slapping against mine seems like a sure fire way to lose wood.