3. g3 is an illogical move, since White's light-squared bishop can already develop via the e2-b5 diagonal. This means that either White's 1. e4 or his 3. g3 pawn moves were wastes of a tempo (move). Therefore, White has lost a move in which he could have developed another piece, and is giving Black the chance to take a lead in development by bringing out one of his own pieces (e.g. 3... Bc5) In particular, if White chooses to develop his bishop via the e2-b5 diagonal, he will leave undefended holes on the light squares of his kingside, which will be a major weakness in the middlegame, particularly if he chooses to castle on the kingside.