horrendously bad luck if this was true of all 3 of your flights, but another explanation is FAM on those flights - they are required to assume specific seats in the F cabin. however, since your MIA-ATL was on a different day, this is an unlikely explanation for why all 3 segs would have been DG.
i'd definitely call and escalate, that is completely unacceptable without a very good explanation and probably some comp to offset.