Ian,
> Should I use mid high absorbers only on the apex part <
The general goal of alternating bass traps and mid/high panels still applies. You could mount the panel traps sideways and place them near the bottoms of the 3 foot walls, and also near the top where the angled walls meet. That puts the bass traps closest to the room corners where they do the most good. Then use some mid/high absorbers on the main portion of the angled walls.
--Ethan