There are a couple of ground rules that might help you get started:
1. Be careful not to be lured into the trap of "they have the same primary ability stat, they should be great together". There are a good chunk of classes that, while they have similar requirements, just don't work together well. I would say that applies to pretty much all instances of Wisdom classes. BG3 monk may break that mold a little bit for a Dex based Ranger, but for the most part there is either so much overlap that you might as well stick with the single class (Ranger/Druid), or an oil and water situation like Moon Druid and Monk, where one's primary specialty can't be used at the same time as the other's.
There are plenty of times when it is a good rule to sync ability scores, just don't assume it is a perfect match every time.
2. When mixing martial classes, with each other or spell casters, make a very conscious choice of when to aim for 5th level. Extra Attack is a big leap forward in their capabilities and too much, or too early, multiclassing can drastically undercut their performance.
3. In general, mixing primary spellcasters together isn't amazing. Sorcerer is probably the most worth multi-classing because of the metamagic (in tabletop you can grab a feat for that though). Otherwise though, you should be thinking about what special abilities your are gaining that are worth trading off your higher level spells.
4. There are a couple classes that mix well with just about anyone and some that are better left alone. A couple levels of rogue, fighter, or cleric can really help a class that is going to drop off in the middle to late levels. Paladins are a prime example of a class that drops off. I think most people agree that past level 6 or 7 (depending on the subclass) it is much more effective to multi-class a Paladin than to stay single class.
Other classes are terrible for dipping because their powers rely on their levels in that one specific class. Druids need druid levels to increase their wildshape power, so multi-classing with them should be done with great care. Monks, and to a lesser degree sorcerers, can be gimped by multi-classing because they lose out on their unique point wells (Ki or sorcery points respectively).
5. Specifically for BG3 and other CRPGs, keep an eye out for too much overlap between characters. It's easy to keep an eye out for when you are all single classing. But once you start multi-classing you may end up hurting your overall damage by dipping everyone into a support/healer class 'just in case', or turning all your casters into sub-par multi-role close combatants.
These are all general guidelines, and there are exceptions to all of them, but hopefully this is a good foundation for you before you delve into sub-class specific considerations.
