You're mostly correct. 85% assumes that every game has a 50/50 shot. Vegas odds alone tells us this isn't the case. Some teams are more likely to win any one game over another. Using current Vegas odds and the 3 games in question (Pitt/Balt, Tenn/Jax, Buf/Miami), the games can be adjusted to show that a result that hurts the Bills has a percent chance of 59.4%, 65.8%, and 45.5% respectively. Multiplying those out to get the odds that exact sequence happening and the Bills missing is 17.8%. That means the chances of Bills making is %100 - 17.8% or 82.2%, lower than your suggested 85%. This also assuming that no ties happen, which is a safe assumption.