Ring of Protection +3

Increases the armor of the Hero by 3 when worn.

Source
Arthas and Muradin Bronzebeard find one in Frostmourne, chapter 9 of the Warcraft III human campaign, The Scourge of Lordaeron.

Arthas and Kel'Thuzad can also find one in Blackrock & Roll, Too!, chapter 6 of the undead campaign, Path of the Damned.

Grom Hellscream finds one in The Hunter of Shadows, chapter 5 of the orc campaign, The Invasion of Kalimdor.

Illidan can find one in A Destiny of Flame and Sorrow, chapter 6 of the night elf campaign, Eternity's End.

Malfurion, Tyrande, and Maiev can find one in Balancing the Scales, chapter 5 of the Frozen Throne sentinels campaign, Terror of the Tides.

Tyrande and Maiev can find one in Shards of the Alliance, chapter 6 of that campaign.