Not at the same time. No 3s LiPo batteries ever. Stock ESC is made to only run 2, 2s Lipo batterries.
No. I run my Summit on 2+3s with no issues. Motor will not last as long as on 4s for sure, but 5s voltage is about the same as two fully charged 7 cell nimhs. No problem for the EVX-2. Just be sure to plug the 2s to the BEC side of the esc, turn off lvd and use a lva.