I am trying to build a Human Wizard 3/Rainbow Servant 10/Initiate of the Sevenfold Veil 7, but for this I need to be able to cast 3rd level Arcane spells at level 3. What combination of feats should I use for that? I've looked at Sanctum Spell, but that one screws you over the moment you step out of Sanctum at level 4. Is it possible to pull this off without Sanctum Spell?
Mostly taken from this answer, but I’ve stripped out the references to things that won’t help you.
By the way, I strongly recommend that all DMs ban all of these tricks, and furthermore ban the build that you’re proposing, should you find some other way to accomplish it.
Improved Sigil (Krau) Illumian: race and 1 feat (Races of Destiny)
An illumian can take Improved Sigil (Krau) to apply a free Heighten Spell to a number of spells. Just take it at 3rd level, pick a 2nd-level spell, and you’re good to go.
Precocious Apprentice allows you to cast a single 2nd-level arcane spell, if you wanted to try to do this as a Wizard 1 rather than Wizard 3. Requires flaws though.
Earth Spell: 3 feats (Races of Stone)
Earth Spell requires Heighten Spell and Earth Sense, basically allowing you to have a free extra level when using Heighten Spell. Heighten 1st-level spell to 2nd-level with it, and you get a 3nd-level spell. Three feats that early requires being human, or flaws.
Precocious Apprentice doesn’t help here because you can’t prepare your heightened 1st-level spell in the Precocious Apprentice spell slot; you can only prepare the chosen spell.