The development status of Half Life 3 has been a mystery for more than a decade now and there is still no sign of the game coming. Even so, fans can be certain that Half Life 3 will see the Combine being forced into a full retreat.
Like how it was portrayed in the fan-made Half Life films, Half Life 3 will look to continue with Alyx reuniting with the G-Man and protagonist. Meanwhile, the Combine had just acquired the Borealis and is learning on how to make it work.
With that being said, it will be up to the player to stop the Combine and destroy the Borealis once and for all. When that happens, the Combine will retreat as they see no use for being on Earth. Thus, this will also end the Combine war and restore the peace in the universe.
As mentioned before, the above plot is simply based on rumours and a decade-long thought. Fans see no way for Valve to avoid from applying the above plot in Half Life 3.