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.
Source: TPCi Rules Team (2019-05-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM8_106