Best I can tell from the information available this the way it is going to go.
The definite part is if the Marlins tie with the Astros the playoff game will be in Florida. Should the Marlins tie with either the Phillies or the Nationals then the Marlins become road warriors.
If there is a three-way tie and two of the teams tied for a division title then the two teams who tied for the division will play a one game playoff to determine the winner of the division and the loser will play in the Wild Card playoff.
If there is a three-way tie and none of the teams tie for a division title then the team with the best overall record against the other two gets the option to choose whether they will be team A, B or C in the playoff rotation. Team A will host team B in the first round. The winner will host team C.
So there you have it.