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I have finished the first and middle layers on a Rubik's cube and now I would like to complete the yellow cross on the third layer. Every instructions manual I have seen assures that there are one of two possibilities: either no yellow edges on the top layer (only yellow dot in the middle) or two yellow edges on the top layer (it is a line or an "L" shape). However my Rubik's cube has three yellow edges on the top layer.

Firstly I thought that it is possible to apply the common algorithm: F U R U' R' F', but this gave me no result.

I also tried to switch the edges (I thought that this may reduce the number of yellow edges). The result was either three yellow edges or one yellow edge. Both cases doesn't occur in any instructions.

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    $\begingroup$ Did your cube pop once by any chance and was put back together incorrectly? Or someone pranked you by flipping an edge? Having a single rotated edge is indeed not possible on a regular 3x3x3 Cube, which is why you are unable to find any algorithms for it in tutorials. $\endgroup$ Aug 21 at 17:14
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If you are absolutely certain that all 8 edge pieces not on the top layer are in their correct places and in their correct orientations, (this seems to be the case, since you say you have finished the first 2 layers) then there are exactly two possibilities:

  • either the number of correctly oriented edge pieces on the top is even (0, 2 or 4), or
  • you have an unsolvable cube.

In the latter case, your only option is to break the cube apart and reassemble it. To do that, twist the top layer by 45 degrees, push a flat head screwdriver (or some other suitable utensil) into the gap under an edge piece of the top layer, and twist the screwdriver lightly to pop the edge piece out.

The rest of the pieces will come apart easily after that, and since your cube can only have reached the unsolvable state by being disassembled and then assembled incorrectly, it's probably a good idea to take it completely apart (or at least until you have disassembled all the unsolved parts): in addition to the single-edge flip you seem to have encountered, there are two other possible issues (twisted corner piece and/or a two-piece swap) that can cause a cube to be unsolvable. Reassembling the cube into the solved state is by far the easiest way to make sure you don't run into any of those.

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