1. Pokémon Moon - 3DS2. Tony Hawk's Underground - GCN3. Warhammer 40,000 Dawn of War II: Chaos Rising - PC4. Warhammer 40,000 Dawn of War II: Retribution - PC5. Disgaea: Afternoon of Darkness - PSP6. X-Wing: Imperial Pursuit - PC7. Star Wars Republic Commando - PC8. X-Wing: B-Wing - PC9. Blazing Lazers - TG-1610. Tales of Xillia 2 - PS311. Shining Force CD: Shining Force Gaiden - Sega CD12. MUSHA - Genesis13. Sonic CD - Sega CD14. Final Fantasy Legend III - GB15. Tales of Zestiria - PS316. The Legend of Zelda: Breath of the Wild - Switch17. Horizon Zero Dawn - PS418. Tales of Berseria - PS419. Battlefield 1 - PC20. Turok 2: Seeds of Evil - PC21. Mass Effect Andromeda - PC22. Starflight 2 - PC23. Armored Hunter Gunhound EX - PC24. Space Megaforce - SNES25. Persona 5 - PS426. Torment: Tides of Numenera - PC27. Cosmic Star Heroine - PC28. Prey - PC29. Strafe - PC30. Mystic Origins - NES31. Fire Emblem Echoes: Shadows of Valentia - 3DS32. Ultra Street Fighter II - Switch33. The Legend of Heroes: Trails in the Sky - PC34. Ultima IV - PC35. Environmental Station Alpha - PC36. Dust: An Elysian Tail - PC37. Hollow Knight - PC38. The Legend of Heroes: Trails in the Sky Second Chapter - PC39. The Legend of Heroes: Trails in the Sky the 3rd - PCFinished up the Trails in the Sky trilogy. The 3rd is both completely unnecessary and utterly vital, all depending on your point of view. A good descriptor I saw for it is that it is the Appendices to the Lord of the Rings. The main plot was wrapped up prior to it, but there's still a whole ton of world and character information contained within. And personally I think it is very worth playing for those parts.
The 3rd starts six months after SC and is focused on Kevin, the priest who joined the party midway through SC. It's mostly delving into his backstory, as well as fleshing out the Church that has an omnipresent and rather powerful presence in the world. Shortly into the game you end up getting sucked into a world that is made up of equal parts recollections of past areas and new featureless dungeon. So you don't really have NPCs to deal with; it's pretty much straight dungeon crawl with story bits at the climax of each area. You will soon be joined by the rest of the gang from the first two games, and a few of them end up having a bit of character building themselves in certain key areas.
However, if you only do the main plot you are missing out on a ton of what makes this game valuable. Equally important are a series of doors that will transport you to events of the past. Most of these have you as a passive observer, but a few are fully controlled by the player. Every character gets their spot in the limelight, and these end up being a mixture of filling in backstory, adding in general world story, or looking at what happened to characters after the events of SC. In the process, several threads for the larger plot of the universe that will be picked up in Zero no Kiseki and Trails of Cold Steel. You might think of it like Trails in the Sky encompassing Rogue One and A New Hope; one major threat is dealt with but there is far more going on in the overall universe. I definitely wonder just how much Falcom has plotted out at a high level.
The major gameplay addition is that Time, Space, and Mirage have been added as weakness/resistances for enemies. This gets shown off in the early areas by a bunch of enemies weak to one of those three. The spell lists for Space and Mirage were filled out a bit better in order to take advantage of this. But since you're starting at such a high level (you start at 95ish, end around 130ish) you'll find yourself sticking with the rank 1 magic due to how the damage formulas work out. Like before, you really don't need to engage in too many fights. The end game is a four part dungeon where the party splits up into four teams, so you will want to have them all kitted out in quartz, and that's going to be the majority of your ginding. But even that's not too bad; I did it over the course of two nights (a couple hours each night).
So if you've gotten to the end of SC and are wondering why you should play The 3rd, the answer is "how much do you care about the characters and the world?"