The rules in Baldur's Gate 3 are largely those of the pen & paper RPG Dungeons & Dragons 5th Edition
I‘m coming to the game from having played a lot of pen&paper D&D, and the older Baldur‘s Gate games, which as I recall it were a bit more lose about the rules compared to the old D&D rules.
In any case, because for this one, the rules seem to adhere very closely to those for the tabletop RPG, both for actions, attack rolls etc in combat, as well as for character development, I‘ve been thrown off a few time where they differed.
For example, crossbows have no limit to number of shots per round in BG3 (which in the paper version is one of the balancing factors for their generally higher damage than bows). Magic items seem to lack attunement, which in the paper version limits how many strong items a character can use at the same time. Concentration spells that in the paper version often are only good for a minute (10 combat turns), or a short perid of 10 minutes often last for the whole day, as long as concentration is not broken (e.g. expeditious retreat).
What are the main discrepancies from the written 5e rules, where things were implemented slightly different in the computer game?
Best Answer
Raven Dreamer’s answer gives a good overview, but if you’re interested in an exhaustive list of changes, one is available via the independent Baldur’s Gate 3 wiki project. The main article covering changes to basic rules like initiative, healing, resting and more is “D&D 5e rule changes”, and there are also further articles for changes to races, classes, spells and feats. These articles all assume some familiarity with standard 5E rules. These pages are also updated when patches make further changes to the rules used by BG3 (especially ones which fix bugs preventing a rule from operating as it does in the tabletop game).
I would add that there are some changes that are not obvious but have a big impact on the game. As one example, initiative is not a standard Dexterity check, but instead uses a d4 (though the Dexterity modifier is still added). This makes it more likely player characters will end up next to each other in the initiative order, and such characters effectively act simultaneously, able to take their actions and movement in any order they like. Like many of the changes to a basic rule, this has many knock-on effects, such as modifying how many bonuses to initiative work (advantage on initiative rolls is generally changed to a flat bonus of +3 to +5, for example).
A few other significant changes which surprised me and affect power level:
There are many more, but those give you an idea about the extent of the changes.