I have been playing a little killzone 2 multiplayer now (I think im rank 3) but I can still only pic the soldier class. How do I go about unlocking different classes? Is it a badge thing?

It's just a ranking up thing. You're rank three, so you probably have what? About 200 rank points? Once you get 350 rank points, you will unlock the Medic i believe. Then 500 rank points gets you more guns. 800 rank points unlocks the Engineer. Etc.

Here is a decent FAQ that shows all that info:

http://faqs.ign.com/articles/958/958214p1.html

If you just want classes, just join Death Match games...you gain about 20-30 a round, if you win you get the 1.5 boost.

too many people are asking this question, just give it time