It really does depend on what you want to do later on. My third plane is a Sky Raider Mach II because it's very inexpensive and a very fun (and fast) aerobatic racer. I got it simply because I wanted something inexpensive that I could throw around the sky and try new things with.

If you wanted to move on to precision pattern flying, you'd want a different third plane than you would if you wanted to try hovering and harrier rolls (3D).

I'm just in it for fun, so I'm going with planes that look like fun. That's why my fourth will most likely be a Sig Hog-Bipe. I've always liked biplanes and that one looks like fun!