I would suggest doing heavy dips, close grip bench, French press/Skull Crushers, Overhead Extensions and Pushdowns. All of those are great for your tri's if your looking for mass. Heavy Dips and Close grip bench being the most important IMO. Which excersises you choose and how you perform them depends on your fitness level. Try not to lean forward as much while doing dips to let your tri's do most of the work. I believe dips will hit the lateral, medial and long heads of the tri's.