"What are the best weight training exercises to increase pec, delt and trap size?"
Here you go-
Pecs: Weighted Dips
Delts: Military Press
Traps: Deadlifts, Shrugs
All of the above exercises tie-in pecs, delts and traps very effectively. Work limited sets of these, work `em heavy and to complete failure, eat and drink high protein and you'll be needing new shirts soon!