You draw three cards and discard two cards all while Cephalid Sage’s ability is resolving. Nothing can happen between the two, and no player may choose to take actions.

If you don’t have enough cards in your graveyard at the moment Cephalid Sage enters the battlefield, its threshold ability won’t trigger. If the number of cards in your graveyard changes and Cephalid Sage loses the ability once it has triggered, the ability will still resolve.