Can a player use Serena's first option if they would still have 5 or more cards in their hand even after discarding 3 cards?
The main effect of Serena's first option is to discard cards from your hand; so as long as you can discard at least 1 card from your hand you can use it.
Source: TPCi Rules Team (2022-11-10)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH12_164
SWSH12_164