I don't know, I am pretty happy with what I have. I think 3 is enough, just change the maps out when you need new ones.
I think there should be another section added in the profile, so under the maps you see the 'custom maps' section which lists the maps you've made so far. Maximum maps that can be saved is to 50, you could change your current game maps by clicking/choosing three custom maps.
I don't think custom maps comes up when you choose random map selection in game. I may be wrong on this, but if not it would be nice. Or offer a random custom map feature where only your custom maps are randomly chosen. 3 slots is not enough. You spend 80 coins for custom map and you get 3 slots for maps. You pay 30 coins for a map set and you get 10 maps. I tinkered with the custom map making and just assumed that once you purchased it you'd get additional slots. Very disappointed when that didn't happen. Never woulda wasted the 80 coins had I known that. While I myself rarely use FFA maps, that option for map design should also be in there. Other map making features would be nice, such as raise or lower whole terrain. Create straight lines or curved lines of terrain. Raise/Lower all tanks. Cutout pieces for making holes, valleys and hills, etc.
