IMHO I think you're fine with those 2 boys, seriously you're trying to teach them to be able to have run of the house without worrying about them right now and 3 may just not be good right now. I never commented about a cat because I think that's just plain uh.
Maybe a goldfish would be good, you can talk to him and make sure his fish food is in good condition and clean his tank once a week and the boys will sit and watch you do it and the fishy will make funny faces at them all day long, doesn't that sound good????!!!!!