Gauntlets of Ogre Strength +3

Increases the Strength of the Hero by 3 when worn.

These thick leather gloves appear at first glance to be far too big to wear, and yet they fit small, medium, or large creatures normally. The gloves make their owner's hands and forearms appear unnaturally oversized, although the owner's dexterity and fine manipulation is uneffected. The cured surface appears gnarled and skin-like, even sprouting bristly hairs on the back of the hands and forearm sections.

Source
Arthas finds one in Blackrock & Roll, chapter 2 of the Warcraft III human campaign, The Scourge of Lordaeron.

Thrall can find one in Cry of the Warsong, chapter 3 of the orc campaign, The Invasion of Kalimdor.

Tyrande can find one in Brothers in Blood, chapter 5 of the night elf campaign, Eternity's End.

Maiev can find one in The Tomb of Sargeras, chapter 3 of the Frozen Throne sentinels campaign, Terror of the Tides.

Arthas, Kel'Thuzad and Sylvanas can find one in King Arthas, chapter 1 of the scourge campaign, Legacy of the Damned.