[RPG] Is it possible to cast 3rd level Arcane spells at level 3

RAW: Ambiguous

The rules are not clear on what, exactly, constitutes an “arcane spellcasting class” or a “divine spellcasting class,” probably because initially it was obvious. By default, assassins, bards, sorcerers, and wizards were the arcane spellcasting classes, blackguards, clerics, druids, and paladins were the divine spellcasting classes. There was no ambiguity: the former are the classes that cast arcane spells, and the latter are the classes that cast divine spells. Easy.

Then they printed things like Alternate Source Spell, Rainbow Servant, Sha’ir, and Southern Magician, which introduced ambiguity that hadn’t existed before. In these cases, you have spell slots from one class being used for either arcane or divine spells.

Does this new feature suddenly make the class into an “arcane spellcasting class” or “divine spellcasting class” where it wasn’t before? There hadn’t ever been a strict definition before, and they didn’t print one at this point, either. Some of these effects seem to try to include wording that prevents this kind of thing, but much of those rules are also unclear. For instance, consider this from Southern Magician: “The actual source of the spell's power doesn't change,” which Customer Service interpreted as preventing entry to mystic theurge. But it doesn’t really say that, does it? It says something about power source, which is unclear.

Unfortunately, there’s no direct, rules-as-written, “as it says on page xyz of Complete Shenanigans” kind of answer to this question.

Recommendation: Never

RAW is ambiguous, but what’s going to work well in-game is not: never, under any circumstances, should one be allowed to advance wizard spellcasting faster than the wizard does. That should never, ever happen in any game, and if you’re going to allow it you might as well allow Pun-pun.

Allowing these sorts of tricks to qualify for mystic theurge, and other prestige classes and feats that require one type of spellcasting or the other, is pretty clearly legal, RAW, and also usually far less troublesome. The only exception I’d be likely to make is the dweormerkeeper from Complete Divine’s web enhancement, but then I’d probably just ban that class outright.

Even allowing a divine-only prestige class to progress wizard spellcasting is almost-always not a problem. It’s the double-progression that should never, ever happen.

The table is always abbreviated and for reference only. The “real” rules are the text. The errata rules specify this explicitly. So you have to go by the text.

And the text in this case talks about caster level, spells per day, and spells known, not manifester level, daily power points, or powers known. And the default magic–psionic transparency says nothing about (prestige) class features. So talk about caster level, spells per day, and spells known in a class feature does a manifester no good (as opposed to those things in a spell or magic item, which do aid a manifester).

So no, officially, these do nothing for you. They’re usually easily-translated, and some even have official Adaptation sections suggesting it, but then the arch-psion is also pretty easily translated and you already said that was out.

The only other thing you might be able to do is somehow get access to the magic mantle from Complete Psionic, either by dipping a level of ardent or, if you’re specifically an egoist psion, by using the true healer variant to get the life mantle, and then taking the Tap Mantle (magic mantle) feat at 6th and the Don Mantle (magic mantle) feat at 9th (both Tap Mantle and Don Mantle are from Complete Psionic). This might accomplish what you want because the magic mantle is worded terribly and so arguably allows this kind of “complete transparency” over and above what Expanded Psionics Handbook suggests. This is almost-certainly not intended, since Complete Psionic describes the effect as something “Most campaigns already [do],” but ultimately the wording is so poor that no one can say for sure. Only your DM can answer that question for your campaign.

Were it me, not in a million years would I want to invite magic mantle abuse into my game; I would much, much rather perform some minor translation for you.