The Triforce is from The Legend of Zelda series—it does not exist in Warcraft III.
If you're playing a custom map (DOTA, RPG, ORPG, Tower Defense), the creator may have added a "Triforce" item. Check the map's quest log, item tooltips, or forums (Hive Workshop, Epic War).
Specify the campaign, chapter, or custom map name if you're hunting a specific item.