Your heating problem is two fold:

* you ran the cells to the cut off
* the cut off was set at 3V

3v for a lipo is as good as dead - when they are this low the internal resistance is very high and amp draws rise because voltage is down , add that extra burst of heat to that already in the pack and you have an - you get the performance you pay for explanation.

Either set the cutoff at 3.4v or run it to a time limit.

AND ThunderPower 30/60C cells against zippy and rhino etc - they are slightly more expensive give much more capacity and in the second half of the pack have much better voltage - the 45/90's are the bells cell around at the moment.
olty on a 3 v