It’s not legal to have any form of MP3 player on a triathlon course, so you should be used to not training with one. It becomes kind of a crutch to help move things along, but where is that crutch come race time?
Using a swimming MP3 player is a waste of money. How it was explained to me is the most popular model works by vibrating against your cheekbones. In 26 years of competitive swimming I’ve never had the desire to listen to music. You’re not in the water that long anyway. Lastly, if you’re swimming an actual workout with a bunch of sets, it’ll be stop and go the whole practice.
Listening to music while riding a bike is dangerous; I wouldn’t even call it safe to do if you left one ear open. There are a lot of important sounds you miss because of this. There have been studies done where you’re listening to music in one ear and someone is telling you important information in the opposite ear. The subjects don’t remember most of what they were told. How is that going to work out when it comes to riding on the road? Did you hear that car back? Probably not, the music is more than likely drowning out the sound of their engine.
Remember, if you can’t race with it – you shouldn’t train with it.