4. Position the 3rd Layer Corners
The goal of this step is to partially solve the corners. You want them in the right place, but not necessarily flipped the right way. You move the corner pieces around with Move 3, which swaps the top-right corners.
There are always 2 corners that can be placed correctly but they can be hard to find. Check the corners: if you don't find 2 in the right place, turn the top layer a quarter turn and check again. Keep looking until you find them!
1. If 2 neighboring corners are correctly placed, hold the cube with these corners on the left to do the move.
2. If opposite corners are correctly placed, you will have to do Move 3 twice, the first time with any face forward. Then put the good corners on the left and repeat the move.