While resolving Starseer Mentor’s last ability, the target opponent chooses a card to be discarded without revealing it, chooses a nonland permanent to be sacrificed, or chooses to do neither. Then that player discards that card, sacrifices that permanent, or loses 3 life. That player can always choose to lose 3 life, even if they have cards to discard or nonland permanents to sacrifice.

Starseer Mentor’s ability cares whether you gained or lost life this turn, not how your life total changed. For example, if you gained 2 life and lost 2 life in the same turn, you still gained and lost life that turn, even though your life total is the same as it was at the start of the turn.