I think less frequent with more intensity is the best route to go. It's easy to overwork the triceps. Any pressing movement heavily involves the triceps. So you're getting triceps work during your chest and shoulder training, and then some people like to work their triceps on a completely different day. I started noticing more fullness when I dropped the frequency (I was doing triceps apart from chest and shoulder days, so that's 3x per week) to twice a week. I would add in some triceps specific work on either my chest or shoulder day. Since my chest is a weak point for me, I usually wait until shoulders day to train them.
I generally don't do any more than 2-3 exercises (usually 2) for specific triceps training, and around 3-4 sets per exercise, so that's about 6-8 total working sets for triceps, on average, for me. If you're trying to gain mass, the key is to use heavy weights. You may see triceps development from lighter, isolation exercises (which I don't completely avoid), but if you want more size and fullness, go heavy.
Some good mass exercises for triceps include:
- Close grip bench presses
- Parallel bar dips (weighted if possible)
- Bench dips (weighted)
- Extensions (not really a compound movement, but you can go heavy)
Really stretch your triceps out after you work them, regardless of when you're working them. After training the chest or shoulders, don't neglect to stretch out your triceps.
Also, if you really want to start seeing more size and fullness, concentrate on heavy compound lifts such as squats, deadlifts, presses, ect.. I know it's preached around here daily, but you really do see overall body results from doing heavy squats and deadlifts (albeit you're doing them correctly with adequate rest and diet).
Train hard brotha!