Eegads, we can debate which system of theory we most ascribe to but: in ANY system used and validated by any academic music theorist after about 1920 or so, there's no such thing as a "third inversion." I myself am primarily of the Schenkerian school, wherein the concept of "inversion" only carries to the FIRST, i.e. the 3rd of the chord in the bass. A so-called "second inversion" is a dissonance, creating an interval of a fourth in the lowest two voices, which is not a consonance (i.e. a stable harmony). It is a dissonance that must resolve. I know, we commonly refer to 6/4 "inversions" but in Schenkerian and other systems of explaining Western tonal music, they are not truly thought of as "inversions." Thus, certainly ANYTHING with the 7th as the lowest voice, or above (9th, 11th, etc.) in the bass, is a dissonance requiring resolution and not an "inversion."