If you Heedless Resurrection on a Elnuk played from hidden, how does it resolve? does the Elnuk recycle itself from the trash and do you get the +3?

No on both — once Elnuk Herd has left the board, its play-from-Hidden trigger can't do anything.

Sequence: you play Elnuk Herd from face down for [0], and its "When you play me" trigger goes on the chain with its +3 target locked to another friendly unit at the battlefield you hid it at (Rule 811.1.d.2). In response you play Heedless Resurrection, killing the Elnuk as its additional cost (Elnuk is 6 Energy, so the unit you bring back from trash must cost no more than 6 Energy and 0 Power — and it can't be the Elnuk itself, since targets are chosen before costs). When the Elnuk's trigger resolves, the card is in your trash, so the ability can't reference the unit to determine which battlefield it was hidden at — its instructions are ignored. That means no +3, and no recycle (from the trash or anywhere else). This is the same reasoning as FAQ #12771.

If you instead meant playing a different Elnuk Herd from your trash with Heedless, that's the opposite case: you do get the +3, because it was played from the trash rather than from face down so the "recycle me" clause is off, and the same-battlefield targeting restriction doesn't apply either.