It takes a special kind of athlete to take on a triathlon. Who looks at an Olympic sport and thinks "one just isn't enough for me, I think I'm going to do three of them back to back"? Typically composed of a swimming event, a cycling event and a long-distance run, triathlons are real endurance events, and only the strongest and fittest need apply. We think we'll leave all of that exercise to the professionals!
It seems unnecessarily cruel, then, to make the word any longer than it needs to be, given that the events are long enough already, but some people do. There is no 'a' after the 'h'. This is not a "try-ath-a-lon". It's just a "try-ath-lon". Three syllables for three parts of the race - that should make it nice and simple to remember!