I have the Halcyon 27lb bladder, ss backplate, crotchstap and harness, and tank straps.

If I buy the explorer wing , what else do I need to hook up dual tanks>? I will be towing a 3rd tank as well eventually and what lb lift bladder should I get? I am a firm believer in less is more.

I believe you need tank bands, a manifold, a isolator valve, a Explorer wing made for twins.

3rd tank as in a PONY, or as in a stage or deco bottle?

Depending on if you are DIR or not, stages and deco bottles get mounted on the left side via chect and waist D-rings.