All the factory Arrma 3s stuff I've seen was IC5. Any 3s Lipo with an IC5 connector will work.
I'm not sure of your level of knowledge on this stuff but if your not familiar with Lithium Polymer batteries I would strongly suggest reading up on them before you buy anything. There is some basic but very important knowledge that you need to prevent fires from occurring.
There are a lot of threads on here already about how to care for lipos, how to charge and such. Not really complicated but very important.
I think you better measure battery tray for correct size battery .Id recommend a 2 cell max . Find what motor is rated at as in cells and kv along with esc. those are Ic5 battery connector