Among the following oxides of 3d elements, the number of mixed oxides are ___________.
$$Ti_{2}O_{3}$$, $$V_{2}O_{4}$$, $$Cr_{2}O_{3}$$, $$Mn_{3}O_{4}$$, $$Fe_{3}O_{4}$$, $$Fe_{2}O_{3}$$, $$Co_{3}O_{4}$$

We need to identify how many of the given oxides of 3d elements are mixed oxides. A mixed oxide behaves as if it is a combination of two different oxides of the same element, meaning that the metal exists in two different oxidation states within the compound and can be thought of as a stoichiometric mixture of two simpler oxides.

For $$Ti_2O_3$$, titanium is solely in the +3 oxidation state, so this compound is a simple oxide rather than a mixed oxide.

Considering $$V_2O_4$$, it can be written as $$2VO_2$$ with vanadium uniformly in the +4 oxidation state, which also classifies it as a simple oxide.

Likewise, $$Cr_2O_3$$ features chromium exclusively in the +3 state, confirming its status as a simple oxide.

In contrast, $$Mn_3O_4$$ is a mixed oxide since it can be expressed as $$MnO \cdot Mn_2O_3$$ and contains manganese in both the +2 and +3 oxidation states.

Similarly, $$Fe_3O_4$$ is mixed because it corresponds to $$FeO \cdot Fe_2O_3$$, with iron present in both +2 and +3 states.

On the other hand, $$Fe_2O_3$$ has iron only in the +3 oxidation state, making it a simple oxide.

Finally, $$Co_3O_4$$ qualifies as a mixed oxide because it is equivalent to $$CoO \cdot Co_2O_3$$, where cobalt exists in +2 and +3 oxidation states.

Therefore, the mixed oxides among the list are $$Mn_3O_4$$, $$Fe_3O_4$$ and $$Co_3O_4$$, giving a total count of 3.

The answer is 3.