The only thing I can think of that you'd need five for is a few of the demons that appear banished until you interrupt all five beams pointed at them, but there are a bunch with two or three beams as well, you really don't need those.

I'm not boxing there yet myself as I always like to experience new content solo/together first, but I haven't come across anything that me and my significant other can't tackle together (hunter and warrior combo) that doesn't actually require a large(r) group anyhow such as Kazzak or the Savage Whale Shark. So I'd say you'll be perfectly fine with three, particularly if they're hunters (which I suspect they might be).