The HP50 are the most neutral sounding of the three, the P7 are the most fun sounding and the Momentum is somewhere in between but with a much smaller soundstage than the other two. If I had to have only one as my only headphone I would go with the HP50 for its neutral sound, but I actually enjoy the P7 more, if that makes sense.
In terms of comfort the P7 work the best for me, but the HP50 are very good as well. In comparison the Momentum are smaller but still comfortable for my ears.
In terms of looks the Momentum are my favorite with a very slim profile. The P7 are chunkier but still look great, they don't really look chunky once you have them on. This is where the hP50 falls on its face IMO, they're absurdly ugly on the head, maybe it's just worse for me because I have a somewhat narrow noggin and keep very short hair if any at all. I'm not terribly worried about my image and what others think of me, but I do have a healthy level of self awareness and there's not a chance in hell I'd wear the HP50 in public. It may sound silly, but for me the looks are a deal breaker for these being a portable can.
In terms of build quality and materials the P7 has that in spades. The other two are well built and the Momentum in particular feels like a premium product, but the P7 just exudes quality and luxury.
Right now my inclination is to keep all three but we'll see how much time the Momentum gets. I'll be using the P7 for portability and at the office, which is where I do a lot of my listening. The plan is to use the HP50 at home but with the other gear here we'll see how much use those actually get.