Two trains of thought on this right now:
1.) The "Earth" we saw wasn't our Earth, and they will discover a planet, and call it Earth, and that will be our Earth
2.) The "Earth' we saw was our planet, and we are thousands of years in the future
Except you are forgetting that when they first showed "Earth", we saw the landmasses, and they were exactly as they are on our Earth. You could easily make out North America, for example. So it has to be 2.
The Earth we saw at the end of season three was clearly our Earth. There were no landmasses seen on the Earth they landed on that looked like they do on our Earth. Although given that they didn't bother to show it, we may never know.
Or the planet they do find, if they find one, may happen to look exactly like our Earth.