I take Klonopin as needed but I've never had constipation from it. Klonopin is more prescribed than the other two because it doesn't effect your muscles/reflexes, etc. as much. I've taken the other two as well but not for a very long time. But as far as taking larger doses of valium, you can't really compare mgs per drug b/c each drug has it's own levels where it's effective. For example, I take 150 mg of my anti-depressant which is mid-level, but many people only take 20 mg of paxil for the same effect.
Similarly, a 1 mg klonopin has the same effect as a 10 mg valium in most people. It's just the strength of the individual drug per mg. not really a stronger dose.