I started on road bikes, then got a fixie for winter commuting (mechanical simplicity) and now am building up a cutter/fu-manchu with b43's because I found myself bombing off drops like I used to do when I was a kid (and consequently i keep breaking stuff off my steamroller ).

As you find your needs evolving, your bikes evolve too. I'm sure I'll end up with more than 3 bikes eventually- as long as you buy based on stuff you do, rather than stuff you want to do, you're gravy in my book.