I do several races a year. I choose to do sprints usually, but will be doing a half Ironman at the end of the year just to see if I can finish.
I have always been a competitive swimmer, so I am always in the top when I get out of the water, but I rarely train swimming. It's all technique, and unfortunately unless you learned it early, you're gonna have to go get a stroke coach and spend the time and money getting good and fast.
I suck at biking. It is routine for me to be out of the water in the top ten and then get passed by 50 athletes in a ten mile bike. I have the wrong kind of bike and not enough training. If you want to compete, get ready to spend 2,000 grand or more on a good tri-bike.
Running...well, the bike effects the run, i found this out the hardway with two years of racing with my seat 3" too high. hip pain, back frozen, even losing feeling in my penis. yowza!
After the small adjustment, I knocked three minutes off of my time.
Also, learn how to transition fast. Look an youtube, you'll find some good stuff.