The Third Fleet was a Federation Alliance Fleet that participated in the Dominion War.
This fleet remained to protect Earth when elements of the Second, Fifth, and Ninth Fleets participated in the retaking of Deep Space 9 in 2374. (DS9: "Favor the Bold")
It remains unstated if this fleet was responsible for destroying the Breen fleet that conducted the attack on Earth.
In early 2375, Admiral William Ross, on behalf of the Third Fleet, requested two more attack wings of Romulan Warbirds from Romulan Senator Cretak. (DS9: "Image in the Sand")