About the only context where native speakers use the construction behind [possessive (pro)noun] back (OP's example #3) is the figurative usage...
If you've got something to say, say it! Don't just go talking about me behind my back!
...which doesn't really mean from a physical location somewhere behind me (it nearly always means when I'm not present to hear you myself).
In OP's exact context, native speakers wouldn't normally include them (example #2), because it's implied by behind anyway. In some contexts, such as We were attacked from behind, the word behind simply means to the rear, at the back, but when used in conjunction with leave / left, it always refers to somewhere the referent subject was previously located (the thing is still where it was, but you've moved).
That's to say, if you left something behind, you must have been with it before (you've moved position, but failed to bring the thing with you). But if you see something behind you, for example, that doesn't necessarily imply you had it previously, and / or were ever at its current location.