You are talking about alot of weight gain. I had 14.5" arms at 170 (with 6 months of training), and currently weigh 221 with 18.5", so that is a 51 ibs gain in weight, and I'm a bit leaner than I was at 170, so its closer to 60 ibs of lbm gain.
In any event, I've found that doing trisets (3 lifts back to back with no rest) and drop sets, all with the heaviest weight I can handle (usually each set in the 4-6 rep range) seems to yield the best gains for my tri's and bi's, as well as training them very frequintly. I added 1" to mine during a 10 week period recently, by doing this, however I had not seen any improvement prior to that in 9 months. Heavy compound movements for the back will help with bi's alot, and heavy presses for chest and delts will stimulate alot of the gains you see in yoru tri's.