Rulings

• If you don't control three or more creatures as your combat phase begins, Leonin Vanguard's ability won't trigger at all. Causing a permanent to become a creature during the beginning of combat step won't cause Leonin Vanguard's ability to trigger.2018-07-13
• If you don't control three or more creatures as Leonin Vanguard's ability resolves, the ability does nothing. However, if you do control three or more creatures as it resolves, the +1/+1 bonus doesn't stop applying if you no longer control three creatures later in the turn.2018-07-13