Do you really want to go with a 3S battery? Due to the design and shape the pack will have to be, it will cause quite a weight disturbance when placed inside your Xmod. Why don't you split the batteries up and go with a 3S1P? That will give you the needed voltage plus a rating boost.
From what I read in your post this is what you're describing:
Was I correct? If so, that's what you want for a 3S battery; each being 3.7V multiplied by 3 that'll be 11.1V.