RE: AXI 2208/26 motor questions...........
I'd see what YOUR motor on YOUR pack draws with a 7x5 APC E... if it is no more than 12A or so at WOT I'd say you are okay.
I see on the Axi webpage that they suggest 8x3.8 for 2s on trainers, then 7x4 on 3s for aerobatics. For 3D they do suggest 8x3.8 on 3s (I assume limited bursts of WOT), but it seems significant that nowhere in their table of suggested applications do they even hint at 3s - just 2s, 7-cell and 8-cell.