Looking for more multi-spell buffs

[Gadora]

So Complete Mage introduced the spells Heart of Air, Heart of Water, Heart of Earth, and Heart of Fire, where two or more being active on a character at once grants them light fortification, and all four being active grants immunity to extra damage from critical hits and sneak attacks.

Dragon Magic introduced Primal Hunter, Primal Instinct, Primal Senses, and Primal Speed, where any two being active grants uncanny dodge, and all four grants improved uncanny dodge.

Are there more spells with similar interactions?

[Anthrowhale]

There is the Unicorn {Heart, Horn, Blood} triad.

It's not explicit, but some buffs are highly complementary. For example, Mystic Shield, Ghostform, and Superior Invisibility combo well. Superior Invisibility achieves "concealing the subject from all senses except touch." Mystic Shield "negates the enhancement bonuses and special properties of magic weapons used against the target." Ghostform complements by removing the ability to touch and making the character immune to mundane weapons.

[Gadora]

Ooh, the Unicorn triad's exactly what I'm looking for. Thank you.

[Fero]

The Karmic Aura line of spells from CM may fit what you are looking for.

[sleepyphoenixx]

Luminous Swarm, Fever Dream and Prickling Torment (all CM) have some interaction when used together, but they're not particularly good spells even then.

I'm not sure if they count for your purposes but there's also Caustic Mire (CM) and Cold Snap (SpC) which increase fire/cold damage in their area and Cloudburst (SpC) which reduces fire damage and increases the damage for Call Lightning to d10's.

[Olive_Sophia]

There’s the Soul of X series (Shadow, Anarchy, Light, Order)

And also Master Cavalier, War Mount, and Phantom Charge for your mount.

[Gadora]

These are all great! Thank you, folks. :)

The Soul of X line is a bit odd, with how law/chaos and evil/good are on different spell lists.

Luminous Swarm, et al., is a bad set of spells, but it's at least somewhat interesting.