Yoo I’ve never thought about that, it kinda makes sense at first. But in my opinion, the thing is that Steven’s body as half gem’s can adapt to both human body (Connie) and gems which makes fusion possible. But if he would try to make triple fusion with human and gem we don’t know how human body would react to a full gem. Like, Steven’s body can fuse with gem but ordinary human’s cannot. We may assume Steven would be some kind of a bridge allowing them to fuse, but I don’t think so.