In the event of a three-way tie between the Mets, Giants and Cardinals for the NL Wild Card, the Mets will play the Cardinals on Oct. 3. The winner will be considered the top Wild Card team.
The loser from Mets-Cardinals on Oct. 3 will face the Giants on Oct. 4.
The Oct. 4 winner will then face the Oct. 3 winner on Oct. 5.
Then, finally, the winner from October 5 will be declared the remaining Wild Card team and advance to play the Cubs in the NL Division Series in Chicago on Oct. 7.
In other words, something like this...
To see how two-way tie break scenarios play out, check out this post from Sept. 19.