This makes it painfully easy to get more use out of your creatures abilities, and is so easy to break that its actually banned in Commander
Commander, modern, legacy, Geralfs Messenger is legal in a number of formats, and his price of $7 reflects that
it was far cleaner to win with
However, if you do control three or more creatures as it resolves, the +1/+1 bonus doesnt stop applying if you no longer control three creatures later in the turn.
Im someone that regularly ties a lot of utility into my land slots, and I expect others to do the same
This can lead into even more ramp with red dragon-treasure staples like Ganax, Astral Hunter and Goldspan Dragon