While there is an option to import choices from Seasons 1 and 2, because of an error I was not able to so I was forced to create a new backstory. What differences this will make for people reading these solutions, I can't say. None of the achievements are missable and you'll get them anyway, but your experience may differ somewhat from mine. I'll keep these solutions as general as possible to account for that.
The final chapter (for me at least) began as Tripp brings Javi and Clementine back to the salvage yard. Pick up the hammer and kill the nearby Walker then look at Mari's tape player. You'll find Mari, then you'll find Kate and Gabe trapped inside a truck, surrounded by walkers. Kill the walkers with LS and the face buttons, then use RT to kill the walker that Tripp is struggling with. Once they're freed the group prepares to leave when...
The final major choice of the Episode is to stay with Clem and fight or flee with Tripp and your family.