Multiclassing in Dungeons and Dragons 3rd edition and its descendants is the worst implemented feature I've never seen in any D&D system. What it needs to do is allow players to despecialize, learning something new instead of just getting better at what they already know. What it actually does is making some classes completely worthless on their own and ruin others if they take a single level that isn't their own.
The much simpler group of the classes is the magic users. Use of magic is very tightly tied to spending every single level the character gets into the same class or a prestige class that learns the same magic. While being one level behind in the cleric spells isn't catastrophic, it already places the character below the other magic users in the campaign. Even worse, the good spells are loaded in the later levels, which handicaps the character until epic levels. With the spell progression of wizard or cleric, a difference of two levels already means one "missing" spell level and one less spell in all the previous levels.
Having established why one group of classes can't make use of the system at all, the classes without magic - namely fighter, barbarian and thief have no reason not to take other classes. You get most out of fighter in the first two levels unless you're really want to fill up several trees of feats. In which case the question becomes do you want more health and or thief's skills. For rogue, the only worthwhile ability that improves consistently from levels is sneak attack, but what good is it if the character doesn't have the base attack bonus to make use of it?
Barbarian is effectively extension of either of the two classes: Apart from skills and sneak attack the class gets everything thief gets and has the same BAB as fighter with more health. Barbarian's rage and Saves are mixed around for a bit compared to the rogue, but I would argue that being consistently decent in all of those is better than good in just one. Individual small things fighter or rogue would get are negligible if the alternative is being all around more better.
Multiclassing in the hybrid classes bard, paladin or ranger I don't consider an issue as they already are weaker versions of other classes with qualities from another. The system that is favoured classes and XP penalty is just so bad I don't even consider them worth talking about.
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What would be good multiclassing then? Well, AD&D 2nd edition has a good system, but the game works a bit differently. Most classes don't get all that many things and it has its own limitations. Regardless, I think it would be better in the third edition than the edition's own. If anything, the system is a little too useful. The post is already quite long, but I feel that if someone else wants to try these, it's better to explain it so they don't have to go look for AD&D books.
What's closer to third edition multiclassing is called dual classing and in that edition it's only available for humans, who in turn can't multiclass. The character gets a new class and stops gaining levels in the previous class forever. Instead they level up in the new class as if they started from level one. They will always be a little bit behind single class characters, which is diminished as the levels start taking more and more experience. Taking a new class this way has a requirement of having the new class' primary ability or abilities at 16. Additionally, the character can't use their old class' abilities until the new one passes the old in level. I think this is where favoured classes would work; The racial favoured class could be what the character can dual class out of.
What is actually called multiclassing does not have an analogue in third edition had its successor added in some supplement as an alternative (thank you for correcting), and something similar was brought back as hybrids in the fourth. The character starts with two or three classes, splitting the experience between them. Significantly slower, the character remains alright in all of the classes. For statistic that increase with levels, they get the best of the classes. The hit points are half or a third of each class I'm not sure if this one would be balanced with the evened out experiences; In the older editions mages would gain levels slower than any other class, while thief is much faster, twice the rate of mage at first. The XP penalty might make sense here.
Further explanation on how the system works, as it turned out to be required, from later in the thread:
In third edition it's the total character level that determines how long does it take to progress in levels. In second edition it's the level in a class that determines it. A fighter takes the same amount of experience to gain level 2, no matter if it's the characters first or third class. Taking a large amount of experience for example, a mage gains the fourth level at 10000 points of experience. The same amount split between two classes, say thief and a mage makes the charcter thief 4/mage 3.
For dual classing, I think you are overestimating the time it takes to gain the old class back, not to mention you keep anything that lets you survive. Again, arbitrarily choosing an amount of experience, let's say I play a fighter until level 3, and then switch to cleric. Level 3 fighter has already 4000 experience, while for cleric reaches the same level in just 3000 experience, which is 1000 points before fighter gets level 4.
Obviously some classes are more compatible with having several classes than others: Generally you don't want to slow down your advancement as a mage even more, but "2-4 levels" behind is hardly the case. You can't really dual class starting as a spellcaster because picking another class stops your progression in the previous class forever. With the way mage is designed, I wouldn't want to switch out of mage after becoming good, but to survive better at low levels. On a final tangent, the bonuses are consistently a little bit higher as ability modifiers go up on even numbers, while in AD&D dexterity starts improving AC at 16, at +1. Which means not being quite caught up in the fighter levels of a multiclass character isn't that big of a deal.
For dual classing, I think you are overestimating the time it takes to gain the old class back, not to mention you keep anything that lets you survive. Again, arbitrarily choosing an amount of experience, let's say I play a fighter until level 3, and then switch to cleric. Level 3 fighter has already 4000 experience, while for cleric reaches the same level in just 3000 experience, which is 1000 points before fighter gets level 4.
Obviously some classes are more compatible with having several classes than others: Generally you don't want to slow down your advancement as a mage even more, but "2-4 levels" behind is hardly the case. You can't really dual class starting as a spellcaster because picking another class stops your progression in the previous class forever. With the way mage is designed, I wouldn't want to switch out of mage after becoming good, but to survive better at low levels. On a final tangent, the bonuses are consistently a little bit higher as ability modifiers go up on even numbers, while in AD&D dexterity starts improving AC at 16, at +1. Which means not being quite caught up in the fighter levels of a multiclass character isn't that big of a deal.
