Why would he want to hang out with both of us at once?

This guy and I are "just friends" (who want each other, but haven't done anything about it yet). He has plenty of other friends too, including another female friend who is way younger than I am. I think he wants to be fwb with both of us, but hasn't had any luck yet. He and I act kind of relationshipish, though we aren't in one. No physical stuff, but we see each other several times a week, work out together, eat together, etc. The past few weeks, he has tried getting us to hang out together, all three of us. Invited me out with them and invited her out with us. We're just friends, of course, but I think it's weird. The girl and I are at least 10 years apart and the guy is right between us in age. Why is he doing this? Comparison? I haven't gone for it yet--managed to avoid it instead.