If I use Nihilego's "Nightcap" attack to copy my opponent's Pikachu & Zekrom GX's "Tag Bolt GX" attack, does it need 6 {L} attached or just an additional 3 {L} attached in order to get the bonus effect?

Nightcap replaces the normal amount of energy required by the opponent's attack with a single Psychic Energy cost. So for Tag Bolt GX, Nihilego would need 4 total Energy (1 to pay for Nightcap + 3 extra Lightning) to get the bonus effect.

Relevant cards