For reasons which have been noted, correctly, in other answers, the power of the Three was ended when the One went into the fire. At that point they were (very, very expensive) jewelry and nothing more. They could not have been used to help.
Likewise, Gandalf's role on Middle-earth was ended. His role -- enemy of Sauron -- was strictly limited by the Valar who sent him to be moral and exhortatory. His personal power was to be used only for personal reasons (to protect himself and friends, for instance.) Sauron was now gone, but his orders to not use his native power remained in force.
Besides the power the Three granted them, Galadriel and Elrond also had native powers which were a function solely of their being. Throughout Tolkien's Legendarium personal "magic" power comes from a person's spirit, and the greater their spirit, the greater their power. (This native power would naturally be lost when a person diminishes their own spirit by evil. E.g., Morgoth, and numerous others including Sauron and Saruman in the Third Age.)
Galadriel and Elrond did not diminish themselves that way, and still retained that personal power after the destruction of the One. (This is undoubtedly what made Galadriel's gift to Sam so effective.)
Why didn't they use that residual power to restore M-e? Three reasons:
First, they were enormously powerful as individuals, but that is a long, long way from being powerful enough to restore a blighted kingdom. They needed their rings just to maintain Rivendell and Lothlorien. Healing Gondor was doubly beyond them without their rings.
Second, they are tired. They fought the good fight for over 6000 years, built their little realms as pale echoes of Elvenhome and Valinor beyond the sea and now they are victorious -- and everything they had built was gone. Rivendell was a just a charming valley and would never be more than that. Lothlorien was a wood which, while pleasant, was just another forest. Their world had turned gray and would never come back. It was time -- past time -- to go.
Finally, they understood that the new Fourth Age was the age of Man and probably correctly judged that humans would more deeply value what they themselves built. Even if they could have restored the Pellanor or Osgiliath or remediated the Morgul Vale, Men would better understand what had happened when power went unchecked if they cleaned it up themselves. I think they realized that letting magic depart as quickly as possible was best.