The Diamondbacks won their season series with the Cubs and lost to the Marlins and Reds.
The Reds won their season series with the Cubs and Diamondbacks and tied with the Marlins.
The Cubs lost their season series to all 3 other wild card contenders.
The Marlins will have a better division record than the Reds, which I think means they would win a 2-way tie with them.
Hope I didn't make any mistakes scrolling through each team's schedule, because this really seems to help the Brewers in the event that the Marlins (or maybe even the Reds) catch Arizona or Chicago (but hopefully not both, which could still leave the Cubs or D-Backs in #6).
The 3-way ties are complicated, but I think the only scenario where it would be a multi-step process is if the Marlins and Reds are both involved, just because of the way the Diamondbacks and Cubs can't compete with them in terms of winning enough season series against the other contenders. But I can't stand the way it's worded:
If the three clubs DO NOT all have identical records against one another, and one team has a better record against Teams Y and Z, then that team is the qualifier. If two teams have identical records against one another and each has a better record against the third team, then these two teams follow the two-club tiebreaker rules to determine the qualifier. Otherwise, the three clubs are ranked by their overall winning percentage against one another, and the club with the highest overall winning percentage is the qualifier. If two of the clubs have identical winning percentages in this scenario, then they would follow the two-club tiebreaker procedure.
Does the bolded part mean the team won the season series against Y and against Z, or does it mean that they have a better overall record in all combined games against Y and Z? Because the Diamondbacks could barely win the season series against both the Cubs and Reds, but if the Reds also swept the Cubs in 13 division games, they would still have a better record in pooled head-to-head games than the Diamondbacks. I think that would only matter if you went to the "overall winning percentage" part, but that only sort of implies that the "more season series won against tied teams" interpretation is the first 3-way tiebreaker. It would just be more clear if they talked about it in terms of winning the season series.