When I play Misty & Lorelei, can I discard one or more of the 3 {W} energy I searched for as part of the 5 cards discarded to get the re-use of my GX attack on this turn?
No. You have to discard 5 cards "when you play this card". That is before you search for the {W} energy cards.
Source: Cosmic Eclipse FAQ (2019-10-18), TPCi Rules Team (2019-10-31)

Relevant cards

Note: All applicable cards may not be listed below.
SM12_199
SM12_199