Samy's will have all you need to check out in person. The thing I like about the Lowepro 15L even over the new 20L is that it is deeper so *far* more flexible than other packs normally designed around an SLR system. It also has a pouch and routing for a small camelbak. At 2.45 pounds I find I can pack a lot of stuff in there and not tip the scales too bad. For example, the one I use for my 4x5 holds the field camera, 6 holders, up to 5 lenses on boards and other goodies. I use the space in between the removable insert to stuff things like step up rings, a cable release and other small thin things.
The 15L for the Blad would easily hold your 500CM with the 80 on it, your 180 and 50 upright and leave you tons of room for backs, film, hoods, munchies and other stuff. The tripod holder on the side is great, I have even strapped a 7 pound metal one on it for short hikes but prefer to use a 3 pound carbon fiber model. Just be smart about packing your gear, stack two A-12 backs on top of each other tall side up with a non-lens cleaning cloth in between them. Try to not bring one trick ponies if you have say, a hood that will work for all three lenses.
These new packs are light empty, that is key to keeping the overall weight down. I did not like the earlier models because they over padded and overbuilt them and weighed 6-7 pounds empty. Hasselblads are *fantastic* to travel and pack with because they are so modular and can be broken down to fit a lot more configurations.