Fyfe Gambit has not been played much at the top level where he simply sacrifices a pawn for a lead in development. White may simply develop his bishops first and try to get back the sacrificed pawn and black at the right moment needs to give back the extra pawn to balance the position.
In the Fyfe Gambit and Philidor CounterGambit, white goes for an immediate sacrifice of the white pawn getting only the advantage of immediate piece advances in the game. If black succeeds to foil the immediate piece developments and is able to stabilize the positions then the endgame is much favourable for black and will be able to successfully get and advantage in the endgame. But this move of 3.d4 is not that recommended at a decent level of a chess game.