I would say Chesapeake Bay Retriever. they love to swim, I think they are dog friendly..you never if the dog is going to like you the most. unless you get a puppy and you bond with it.

I don't know how much research you have done on pit bulls, but they are friendly if raised properly. you have other dogs and you'd have to be very educated on the breed since some don't like other dogs, but if you adopted a puppy..you could have it with your dogs..still if you adopted a puppy they were bred to fight but I think they are the best dogs in the world if raised properly.
I don't know if they like to swim as I want a pit bull someday and I need to be more educated before I adopt one. that is my dream dog. I LOVE LOVE the breed!

rotties are great dogs, friendly..not sure if they like to swim. (I have a mix but she HATES water with a passion!) they DO shed a lot. I don't think you can get a dog that doesn't shed at all unless it's a chinese crested or like a jack russel terrier.

not sure about weimaraners. they are cute, not sure how friendly. I don't think they shed much and I have NO idea if they like to swim!

all these dogs that you are interested in need lots of exercise! are you planning on getting a fourth dog??

I hope I made sense here.