One of the best ways to gain muscle mass is by doing an AM/PM split, provided you can eat accordingly. The reason for this? Not only are you getting more frequency (you'd be working out 3 days per week but you'd be working out twice a day, so really you're hitting the whole body 6 times per week), but also you'd gain what some describe as a "summation" effect by exercising while your protein synthesis is already heightened. If you wanted, you could do the same workout twice a day, or create two different workouts for variety's sake. For instance, in your AM workout you may do incline bench while in your PM workout you might do dips.
This AM/PM split seems to be one of the best ways to gain mass, but as mentioned you will need to eat a great deal of clean food. There is a post somewhere (I think by dkm or Vicious) that details the best way to eat while doing this routine. In any case, anticipate eating at least 1,000 calories over maintenance.