There are 5 O's and 2 M's, although the "moo moo" thing was a coincidence, and that title was just to go with the puzzle that, when solved, turned into "old macdonald had a farm". But yeah I think it's telling us that those things aren't in the themes, especially because some of the previous guesses are explicitly included there. EDIT: now though, what could the three letter word be? It can't be "THE" or "AND", but it also cannot be "BUT" because B and U are not in the theme (that is, if the guesses I gaveare correct - which at least for U and B seem to be the correct ones). EDIT2: "No U" is actually pretty big if correct, that leaves us with A E I O. It also fits with the MOO MOO puzzle in the sense that that puzzle solved to "E I E I O" (no U).