1. Three is not better than one but it's definitely cool to have two more sports to choose from during days when you're not up to running.
2. There are short distance triathlon events you can join before plunging on the longer distances. Some are even done on swimming pools and beginner friendly. In some short races you can sign up without training, just borrow a bike if you don't have one.
3. It probably takes two months of training with a coach before you can swim comfortably if you start from zero. Concentrate on your form and proper technique.
4. Your bike should REALLY FIT. A bike that doesn't feel quite comfortable is not the bike for you.
5. Biking is easy but it takes a lot of time to get comparatively fast with other triathletes. Time on the saddle will dictate your progress.
6. Our triathlon community is not as big as the running world, you'll see familiar faces on races and you'll expand your social circle. From lane mates to bikers on long rides you'll have new buddies guaranteed.
7. In a race I usually go out hard on the swim, steady on the bike, hard on the short run and easy on longer runs. You'll know your next approach once you tried it. All I'm saying is that you should have a solid plan for every distance especially on your race nutrition particularly while on the bike.
8. Accidents can happen on bike training and racing, always be alert and stay safe. Bring money and mobile phone on every ride.
9. Triathlon running is different from a stand alone run, much of it is because of the bike ride prior to the run. Try some brick workouts before a race.
10. Structure your workouts to get the most out of it. At the start prioritize your weakness and when you're improving give a lot of emphasis on the bike; it is always longer and your run depends on it.