I've seen these before. You don't cross your eyes all the way; you sort of have to halfway cross them (if that makes sense), then cross them either more or less as needed until you are seeing 3 copies of the image rather than two; then, you just hold your focus there and concentrate on the one in the center.
When u start crossing the eyes you'll see 4 images. As u continue to cross them further the 2 in the middle will come together and there will only be 3. Focus on the middle one.
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