I wouldn't infuse 3 points up, that's wasteful as you mentioned - I just keep them on the side because last I was aware the higher level gear, as long as it was in your inventory was accounted for in future engrams (could be wrong, that is what I was told) - but I also wouldn't equip a 3 point piece of gear over good rolls.
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[quote]higher level gear, as long as it was in your inventory was accounted for in future engrams[/quote] Drops will even account for stuff in your vault. The game figures out what your highest possible power level is and rewards you based on that.