So I found this interesting article on mlb.com listing the different playoff scenarios in the NL. Here's the ones involving the Cardinals (granted all this may be moot based on how tonight's game is going):
Four way-tie involving Brewers, Dodgers, Cardinals, and Rockies (this scenario happens if, including tonight's game, Brewers go 1-4, Dodgers go 3-2, Cardinals go 4-1, and Rockies go 5-1)
Monday, Oct. 1: Two tiebreaker games
The Rockies and Dodgers would play a game to determine the NL West winner. Los Angeles would host that game by virtue of winning the season series, 12-7.
And the Cardinals would play the Brewers (the site can't yet be determined because the season series is still unsettled as of tonight) to determine one NL Wild Card spot.
Tuesday, Oct. 2: The consolation game
Monday's losers would face each other to determine the other NL Wild Card club. The site of this game would determined by the outcome of the regular-season series between the two clubs involved (intradivision record the next determining factor if the season series was a split, but that's not the case in any of these options).
So here's who would host in each potential scenario:
Brewers vs. Dodgers: Dodgers host (4-3 record in 2018 meetings)
Brewers vs. Rockies: Brewers host (5-2)
Cardinals vs. Dodgers: Cardinals host (4-3)
Cardinals vs. Rockies: Cardinals host (5-2)
Wednesday, Oct. 3: The real NL Wild Card Game
This game is scheduled to be played on Tuesday, but a four-way tie would make that schedule impossible to honor. The winner of Monday's Brewers-Cardinals game would face the winner of Tuesday's consolation game, with home-field advantage determined by the same factors as described above.
If we assume that the Cubs maintain the NL's No. 1 seed over the Braves (and for this four-way tie to even exist, Chicago would have to maintain its top spot in the NL Central), the winner of this game would then proceed to Wrigley Field for the start of the NL Division Series. As of now, the NLDS is scheduled to begin with Game 1 on Thursday, Oct. 4 and Game 2 on Friday, Oct. 5.
This means that there's a scenario that exists in which the Brewers, to use the most extreme example, end their season in Milwaukee on Sunday, play in St. Louis on Monday, play in L.A. on Tuesday, play in St. Louis again on Wednesday and play in Chicago on Thursday and Friday.
Scenarios involving three-way ties
Scenario: The Dodgers and Rockies tie for the NL West title and tie the Cardinals for the second NL Wild Card spot (behind the Brewers)
The Dodgers and Rockies would play at Dodger Stadium on Monday, Oct. 1, to determine the NL West champion. The loser of that game would then travel to St. Louis to play the Cardinals the next day to determine the second Wild Card spot. The winner of that game would play the Brewers on Wednesday.
Scenario: The Dodgers win the NL West, and the Brewers, Cardinals and Rockies all finish with the same record
The Cardinals, Brewers and Rockies would choose/receive A, B and C designations, based on head-to-head records. Club A would host Club B on Monday, Oct. 1, to determine one NL Wild Card spot. Club C would then host the loser of that game on Tuesday to determine the second NL Wild Card spot.
Scenario: The Cardinals and Brewers tie for the two NL Wild Card spots
The winner of the season series (still undetermined, as of tonight's game) hosts the NL Wild Card Game.