In precise writing, you should have parallel structure after the both, so I would not write #3 in a document that was being saved. However, I certainly would write in in an e-mail, and it's completely fine spoken. Your reader and listener will understand that the on applies to both the walll and the desk.
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Grammar GeekIn precise writing, you should have parallel structure after the both, so I would not write #3 in a document that was being saved. However, I certainly would write in in an e-mail, and it's completely fine spoken. Your reader and listener will understand that the on applies to both the walll and the desk. "Post it on" rolls off the tongue