Well, the three you have there now should work great together! Avast, MBAM, and Ad-Aware.
However, as you may have seen elsewhere on the forum, Ad-Aware is aging in it's techniques, and Superantispyware would be a better alternative.
Personally, I just use Avast, MBAM, and Spybot (only for passive protections), but then again that's just my personal opinion. Everyone has their own favorite programs.