But at the end of the day, learning any single one is enough. Just be good in one, because you can get the benefits from being good in one single style. There is no need to learn three styles to be able to use taiji. You just need to put in a lot of time and effort into practising one style.
Then why did I learn three styles?
Because my teacher teaches them. I learnt three styles because I want to be able to help him in his classes. I started out only wanting to learn Chen style, but as I followed him around in his classes, I realise I wasn't of much use, since he teaches other styles that I had not learnt. So while I was trying to help him teach for my own improvement, I couldn't help out at most of the classes.
So when Queen Elizabeth visited Singapore in March 2006, and my teacher's class had to do a taiji performance, I went through a crash course in Yang style taiji so that I could be part of that performance. Good thing I have seen my teacher teach Yang style before and therefore was not totally new to the movements, so I could at least keep up by imitating those around me. But that got me thinking. If I really want to help my teacher, I need to know more, so that I can help more.
That got me started on learning Yang style. And then Sun style.
So while I have learnt to use each style to train a different aspect of taiji, I did not learn the three styles for that purpose. I learnt them so that I can help my teacher.