The three pronged strategy was an attack on three sides of an opponent at nearly the same time so that the opponent would not be able to bring reinforcements to any certain side and eventually crush the opponent by surrounding it.
Example: During the American Revolution, the British attempted a three pronged attack: General John Burgoyne from Canada, Colonel Barry St. Leger from the East, General Howe from the South. Were this to be successful, the American's would be trapped from 3 sides of the British army and couldn't escape by sea due to the British navy and all americans including me would be drinking tea right now. Except it didn't work since General Howe decided to go for personal glory and take Philadelphia.