Being its a rockhopper, I am about 99% sure its 26" wheels. You can put slicks on that and have a decent bike for commuting, depends how far you commute. Triathlon? Probably not if you want to be competitive, at all. You may want to change out the largest ring on the crank for something with more teeth for road, You likely have a 42 or 44 now, maybe step up to a 48? I am going to assume its an 8 speed, and if your cassette doesnt get down to 11 tooth small cog, a new cassette wont hurt, either. Definitely go with a steel fork, maybe a Surly 1x1
? Inexpensive and should be strong... Get some puncture resistant tires, Specialized Armadillos, Schwable Marathon plus, etc. The only puncture I have had in the past 15 years was from a sharp piece of rock going through my supremely cheap walmart slick MTB tires... Removed rock, patched tire, and away I went. Luckily It held pressure till I got home, found it flat the next day.
Unless your doing more than 10 mile commutes, 26" tires are fine. Been there, I was happy with it. You likely wont be very competitive in the bike portion of a triathlon without a dedicated road bike, however. Unless its an off-road triathlon...