Ring of Superiority

Increases the Strength, Agility and Intelligence of the Hero by 1 when worn.

Source
Arthas and Jaina find one in The Cult of the Damned, chapter 4 of the Warcraft III human campaign, The Scourge of Lordaeron.

Tyrande can find one in Daughters of the Moon, chapter 2 of the night elf campaign, Eternity's End.

Kael and Lady Vashj can find one in A Dark Covenant, chapter 2 of the Frozen Throne alliance campaign, Curse of the Blood Elves.