From MLB.com:
4-way tie scenario (for 3 spots)
Scenario: Astros, Mariners, Rangers, Blue Jays tie
Result: Mariners, Astros, Rangers IN; Blue Jays OUT
If all four of these teams finish with the same record, ties would be settled in the following order:
AL West title tiebreaker between the Astros, Mariners and Rangers
Second AL Wild Card berth tiebreaker between Blue Jays and two remaining AL West clubs
Third AL Wild Card berth tiebreaker between two remaining clubs
When there is a three-team tie, if one team wins the season series against both of the other clubs, that team gets the tiebreaker. However, that tiebreaker would not be in play if all three AL West clubs ended up tied. The Astros won their season series against the Rangers but lost to the Mariners. The Mariners won their season series against the Astros but lost to the Rangers. And the Rangers won their season series against the Mariners but lost to the Astros.
As a result, the tiebreaker would go to the team with the best combined winning percentage against the other two clubs. In this case, it would be the Mariners coming out on top and claiming the AL West title. (Important note: The Mariners have not actually clinched this three-team tiebreaker, but in order for the three AL West teams to tie with 89 wins, the Mariners need to win four of their final five games, which are against the Astros and Rangers. And if that happens, at that point they will have clinched the best record in games among the three clubs.)
The same steps would be used to determine the winner of the second AL Wild Card berth between the Blue Jays and the two remaining AL West clubs. Because none of these three teams won their season series against both of the other two clubs, this tiebreaker would also go to the team with the best combined winning percentage against the other two. In this scenario, the Astros would prevail and claim the second AL Wild Card spot. (With no remaining head-to-head games between the Astros, Rangers, and Blue Jays, this one has already been decided.)
The tie between the Blue Jays and Rangers for the third AL Wild Card spot would then be settled using two-team tiebreaker rules, starting with head-to-head record. The Rangers won their season series against the Blue Jays, 6-1, so they’d prevail in this tiebreaker scenario and claim the third AL Wild Card berth.