Depends on the campaign, in mine Gondor's been doing fine, until at turn 150 or so they finally lost Minas Tirith.
Thing about Third Age is.. it's very easy. Every faction has something that completely rapes the opposition. It was actually fairly intimidating to start up Stainless Steel after all the ez-mode Third Age battles, haha.
Elves have archers (kill 3000 orcs with 300 elves, 0 casualties..) and the best units in the game, Mordor has Nazgul and trolls (Nazgul alone can rout entire armies, trolls are basically godlike if used properly..), Rohirrim has cavalryspam, etc..