If I tap 3 runes but don't use them, does the floating energy pass on to my enemy's turn or does it disappear after my turn ends?

Ruling: Unspent Energy/Power disappears after each Draw Step and at the End of each turn (including your opponent's turn and draw phase).

Nuances:

• Energy disappears at multiple points: after each Draw Step, at the end of your turn, at the end of your opponent's turn, and at the end of your opponent's draw phase