complete the game at casual difficulty then thought then heroic to get all towers. FIELDRUNNERS 3?
FIELDRUNNERS 4?
FIELDRUNNERS 5?
FIELDRUNNERS 6?

OMG.

they said they were making a fieldruners 3

Yeah, I was really hooked on it for a long time. Add it to your wishlist...maybe it'll go on sale. $10 seems too high for a 2 year old game that hasn't been updated in a long time, and seems be have been abandoned (on Steam) by the developers.

Aw I love this game I witsh I had it

A combo is when you kill 2 or more enemy units at the same time. Killing 2 or 3 at the same time gives you a "combo KO" and 4 or more gives you a "mega combo". The actual points depends on the difficulty level, and the number of units killed. Ex: 5 units on heroic (for all maps except Sudden Death) gives you 2500 points (500 points per unit). See my website at fieldrunners2 [dot] tkzip [dot] com, and click on "Cracking the Combo Code" in Recent Posts. It can also be found under the Blog Posts menu. It has a lot more detail about how combo points are awarded, and what towers are good for getting them. As you can see, two good examples are the Pyro and Nuke. :-)

how to get combos ?

Yes, combos really boost your score. The higher the score, the more coins you get.

How that work, is it the combo