They’re now 2 seats over the majority
Not quite, Republicans currently have an 8 seat advantage after Santos' ouster, 221 to 113, and after Kevin leaves it will be a 7 seat majority.
It is referred to as a "three vote majority" because in a yes/no vote, Republicans can afford to only have three members vote against the party and still have a measure pass. Once Kevin leaves, though , it will still be 3 votes. There is another Republican planning to leave, though, and if that person leaves before the vacant seats are filed, then we are down to a 2 vote majority.