The third step of solving a Rubik's cube is to get in the edge pieces on the middle layer of the cube.
If you have an incorrectly placed edge piece, you may want to put in the other edge pieces in place and doing so may lift the edge piece to the top, where you want it to be.You can get an incorrectly placed edge piece to the top simply by doing Ri or Li, depending on which side the edge piece in on. Then get the bottom corner piece back in place by following the rules as mentioned in part 2 without putting the edge piece back in place.
Once you have the edge piece placed on the top, have the top color on the opposite side of the same colored center. Perform the algorithm F, T, Fi. Twist the cube until the corner piece on the top layer is matching up diagonally with the center piece on the middle layer. Then perform either the algorithm Li, Ti, L or R, T, Ri. This will place the corner piece back in place as well as put the edge piece in place.
This is the hardest part of solving a Rubik's Cube, but I believe in you to complete it.