Momentum doesn't have a definite, material effect on a game, it's just an underlying notion. It could best be compared to morale on the battle field.
Winning three in a row is going to cause you and your team to feel much more confident in your ability to play, and can give you a psychological edge. But, it could also make you over confident, It's effect depends upon how the player responds to it. But, for one player, whats going on in your head will have a huge impact on your gameplay.
Take a look at Lowell, why is he making ridiculous plays one inning, and biffing plays in the next? . Or what about A-rod when he was in clutch situations. Bad luck? I think not, there's something not clicking upstairs they needed to figure out.