Baldur's Gate 3 is a 5th edition game where the action economy decides fights, and almost every strong build is a build that gets more actions than it should.
That is the lens to read multiclassing through. Not "which class has better numbers", but "which combination lets me do more things per turn".
The action economy, plainly
You get one action, one bonus action and your movement. Everything worth building toward adds to that.
Haste grants an extra action, and it is the strongest spell in the game for that reason alone. Action Surge grants another. A Thief rogue gets a second bonus action, which sounds minor until you pair it with something that uses bonus actions well.
Concentration is the limit. You can only maintain one concentration spell, so a party with three casters all wanting to concentrate is wasting two of them. Assign concentration deliberately across the party.
Multiclass combinations that are genuinely stronger
Paladin and Warlock. Warlock spell slots recharge on a short rest, and Paladin smites spend spell slots. This means near-infinite smites, and it is the most straightforwardly powerful pairing in the game.
Fighter and anything. Two levels of Fighter buys Action Surge for almost any character, and a third gives a subclass. Very few builds are worse for it.
Thief Rogue three levels into a melee class. The extra bonus action turns dual wielding from mediocre to excellent.
Sorcerer and Warlock. Eldritch Blast scaling with Sorcerer metamagic, particularly Quickened, produces enormous single-turn damage.
Straight single-class characters are perfectly viable and the game is not hard enough on lower difficulties to require any of this. On Tactician the difference shows.
Gear matters more than the character sheet
This is where BG3 departs from tabletop. There are items in this game that flatly break encounters, and finding them is worth more than optimising your ability scores.
Items that add damage to every attack, items that grant free spell casts, items that let you act after killing something. The game hides several of these in ordinary-looking merchants.
The practical advice: read everything you pick up, and check vendors after each act. A build assembled around two good items outperforms a theoretically superior build wearing whatever dropped.
Illithid powers are a real branch
Tadpoles unlock a parallel progression track with genuinely strong abilities, and using them has story consequences the game is upfront about.
Mechanically they are free power. Narratively they are a commitment. The game does not punish you as harshly as it implies, but it does notice.
Decide early rather than accumulating tadpoles you never spend.
Long rest more than you think
Camp supplies are abundant and long rests advance story scenes that many players miss entirely by rationing.
Resting refills spell slots and short-rest resources, which means the optimal play is usually to rest after any significant fight. Pushing on with three spell slots to save supplies is how encounters go badly.
The Baldur's Gate 3 walkthrough covers the acts and the major decision points.
