If you want to go to 반대편, you should go back the way you came If you want to go to 건너편, you should cross a road at crosswalk If you want to go to 맞은편, you should look for another building which faces the building where you are now

반대편 A place on the opposite side or in the opposite direction. (<--반대하다:oppose) 맞은편 A place where someone can face you. (<--마주하다:face) 건너편 A place across the road or the river. (<--건너다:cross) You can use all of them to mean "across the street, on the opposite side of the road", which is the most common usage I see, but only 반대편 could be used to say that something is on the opposite side of a building from where you are, for example.