If you are buying the lipo for your Slash, it won't work with a 3 cell on the stock speed controller.
If you have a different drive in it, I don't think you need a 3 cell anyway. My buddy has a mamba max in his and normally runs a 2 cell lipo. He put the 3 cell in once and nearly destroyed his truck on the track due to the ridiculous speeds he was getting up to.