Defense of the Heart's ability will check if an opponent controls three or more creatures as your upkeep begins. If none of them do, the ability won't trigger at all. If an opponent does, the ability will check again as it tries to resolve. If none of your opponents control three or more creatures at that point, the ability won't do anything. The opponent that controls three or more creatures as the ability resolves doesn't have to be the same opponent that controlled three or more creatures as it triggered.

If Defense of the Heart leaves the battlefield while its triggered ability is on the stack, the ability will still resolve as normal.