Baldur's Gate 3 allows players to change their class at any time in the game. This is an incredibly useful mechanic that lends itself to roleplaying, but also encourages players to explore different builds thanks to the low penalty level for trying out different builds. Like Dungeons and Dragons, Baldur's Gate 3 also allows multiclassing through this system.
Players can potentially add a new class to their character each time they level up, with the achievement being awarded to those who rank in 12 different classes. This basically keeps the character at level 1 Baldur's Gate 3's Act 3, though it also makes their dialogue options exceptional. For most players, multiclassing means leveling into two or three classes. Multiclassing can result in an incredibly powerful build, but the wrong combination of classes can also make a character much weaker than it would otherwise be.

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The freedom of Baldur's Gate 3 may result in a weaker character
Multiclassing is a mechanic where balance is everything, as it can lead to incredibly powerful Baldur's Gate 3 builds, but it's also very easy to create characters that don't have enough power, especially for players who don't understand which classes synergize. Mixing incompatible classes or multiclassing into too many classes could result in disaster for the character build, leaving the character much weaker than the rest of the party.
This happens especially to players who don't have an in-depth understanding of each class, how they work, and what they offer at each level. Once you understand them, players can start experimenting with how different classes can support each other at different levels.
A general rule to follow is to stick to multiclassing on no more than three classes, ideally no more than two. Some classes, like Fighter, have a few useful abilities at low levels that allow a character to make two attacks per round, so some players multiclass Fighter as a third class. However, the general rule is to stick to two classes. The limitation that multiclassing brings is that Baldur's Gate 3 has a level cap of 12 that applies to all classes.
Multiclassing potentially prevents a character from reaching the most powerful abilities or spells available to the class, severely limiting them.
Building a stronger Baldur's Gate 3 character with multiclassing
Characters with two classes that synergize well can be movers, which often means their ability scores match, though not always. One of the best multi-class built-ins Baldur's Gate 3 is 10 levels in Storm Sorcerer, the Charisma class, and 2 levels in Tempest Cleric, the Wisdom class. Although it may seem incompatible, the nature of Storm Sorcerer and Tempest Cleric results in a seamless synergy while giving the player access to the most powerful spells and abilities of the Sorcerer class.
Good multiclass builds also don't always level the same amount in both classes, as gaining access to certain spells and features at lower levels of certain classes is often all it takes to create a more powerful build that also gives access to the character. to higher levels of their next class. Although it is possible to create strong characters in Baldur's Gate 3 thanks to multiclassing it is also not necessary. Each class comes into its own without multiclassing, and how they play depends on the player's preferences, playstyle, and roleplay.