The odds you beat them 3 in a row, having already beaten them 2 in a row, are excellent as compared to the odds before the series starts.
If you assume that the Brewers should win 60% of the games between the two teams, then the odds of the Nats winning tonight or any other single game in the series are 40%. Feel free to adjust the % to your liking, the math works in the same way regardless.
If you asked if the Nats could win 3 in a row before the start of the series, the odds would have been 40% to the 3rd power, or 6.4%. However, as already observed, the Nats no longer have to win 3 straight. They only have to win one.
so what are their odds that they win 1 more game? still 6.4%.
The more games the Brewers lose to the Nats the greater the chance is that they win the next game. The Brewers should likely win tonight but if not I would bet the farm on them winning tomorrow with their ace on the mound against our starter who has only made 2 starts in the Majors.