They all mean the same thing and can usually be used interchangeably (though I've never heard drop over before), but in the last sentence, only drop in sounds natural to me. … and thought we would drop by on you . … and thought we would drop over on you .
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Mister MicawberI'm a little confused, Gus. The OP's last sentence sounds OK, and I agree that 'drop over on' is wrong, but then you have used it to change the OP's.I see the confusion. I could have been more clear. I meant to explain that the three phrases are usually interchangeable, with the exception of the last sentence, in which only drop in works