Baldur's Gate 3 is clearly building to a big final fight throughout the course of the game, and everything that you do throughout your entire journey is directly helping out with that cause. Throughout the game, you can help out people for seemingly no reward, but these people can reappear in the final fight of the game assuming that you've fulfilled the requirement to get them on your side.
Word of warning, this list will have a whole host of spoilers since we've going to detail how exactly you recruit these characters.
How To Recruit All Allies In Baldur's Gate 3 Act 3
- Arabella: Make sure you complete all three of her events throughout each of the three acts of the game.
- Armored Owlbear: Save the Owlbear Cub in the Goblin Camp during Act 1, and have him join your camp.
- Counselor Florrick: Save Florrick in both Act 1 and Act 3.
- Dame Aylin: Free her from the Shar Prison in Act 2.
- Dammon: Make sure that you've interacted with him throughout the game in order to fix Karlach's heart.
- Duke Ulder Ravengard: Save him from the Iron Throne, or take Mizora's deal.
- Emperor: Don't free Orpheus.
- Halsin: Rescue him in Act 1 then complete his companion quest in Act 2.
- Isobel: Make sure she isn't kidnapped in Act 2.
- Jaheira: Make sure she survives Act 2 and then you complete the companion quest in Act 3.
- Lorroakan: Betray the Nightsong in Act 3.
- Mizora: Save her in Act 2 then have Wyll renew his pact.
- Night-Fingers Keene: Help her stop the Stone Lord in Act 3, and make sure she survives the entire act.
- Orpheus: Free him.
- Rolan: Make sure you talk to Rolan, Lia, and Cal during Act 1. Make sure he survives Act 2, then find him when he goes to rescue the other two. Complete the Nightsong quest in Act 3.
- Ulma: Don't let Astarion ascend, but still complete his quest.
- Valeria: Free them from the Murder Tribunal after investigating the Open Hand Temple Murder.
- Viconia DeVir: Betray Shadowheart during Act 3.
- Volo: Save Volo throughout the game in Act 1 and Act 3.
- Withers: Just make sure you don't somehow lose him.
- Yurgir: Steal the Orphic Hammer from the House of Hope and persuade him to join you during the fight with Raphel.