Act 3 difficulty is/would be fine actually, what's causing the problem are broken or overpowered game mechanics (e.g. Haste implementation, Illithid powers, nerfed crowd control spells), overpowered gear and easy access to long resting without drawbacks/consequences. Of course it doesn't help that Act 3 is the least polished...
They can't touch long rests. They're too crucial to story progression to mess around with. Honestly, removing paralysis autocrit, changing half orc racial to not grant damage dice on crits and deleting potion of speed would probably fix a lot of the balancing problems. Either way, Balduran greatsword also needs a nerf. Remove elixir of Hill Giant and Cloud Giant Strength. Sorc meta magic also needs a nerf. Too many classes get too many extra attacks and too many extra damage dice from gear.
Scripted bosses and their allies should be immune to Surprise status, none of this ambush cheese bullcrap should be possible. Bosses that are in dialogue and their allies should be damage immune while in dialogue and when combat initiates all your party should be placed in combat regardless of hide/invis status.
