At the beginning of your upkeep, if there are three or more creature cards in your graveyard, put a +1/+1 counter on this creature.
Gixian Skullflayer's triggered ability checks the number of cards in your graveyard both when it triggers and when it resolves. If there aren't three or more creature cards in your graveyard each time, you won't put a +1/+1 counter it.
