NFL teams have to go from a 90-man roster to an 80-man roster by Tuesday at 4pm. Most teams will make those cuts well before then, though.
Take the Ravens for example. They played on Thursday night, then had an off day on Friday, and then hit the practice field again on Saturday. The team made their cuts before Saturday’s practice, like most teams will. They cut these guys before the deadline because if one of the ten players gets injured at practice, the Ravens would be on the hook for a financial settlement. In fact, a lion’s share of the 10 players pegged for release also saw limited action Thursday night against the Redskins because of those financial responsibilities. Many teams around the league have their cut lists filled out before that third preseason game.
While the final 53 isn’t decided by the third preseason game, in all likelihood the final 80 is. Each team still has decisions to make on 27 players, but in contrast to week three, most of those guys will see action in the final preseason game, as most key contributors will be held out of the lineup.