Is it normal to be in a threesome with a father and son?

I've been in a relationship with my boyfriend for about two and a half years. He's almost twenty years older than me. Last year he tells me that his son is coming to live with us. I find out that his son is only three younger than me (he's 22.) It was awkward at first, but eventually we hit it off really well. We have a lot in common and started hanging out and the like. One night we got drunk and fooled around. We kept doing it for a few months, behind my boyfriend's back. But we started feeling guilty so we told my boyfriend what was going on. Turns out he knew, but it didn't really bother him because I was still with him as well. We talked about it further and they agreed that they would share me. I take turns spending the nights in their beds (we've never all three done anything sexual together at the same time.) My friends think I'm loco for being involved with a father and son. But to me it's not any different than any other threesome or plural partner relationship. I know I'm not the only one in a perfectly normal well functioning Ménage à trois, right? This seems perfectly normal to me but maybe I'm wrong...