If $\overrightarrow{a}, \overrightarrow{b}, \overrightarrow{c} \in D^3$ it is proved that a + b + c = 0 => a x b = b x c = c x a If $\overrightarrow{a}, \overrightarrow{b}, \overrightarrow{c} \in D^3$ it is proved that $ \overrightarrow{a} + \overrightarrow{b} + \overrightarrow{c} = 0 \Rightarrow  \overrightarrow{a} \times \overrightarrow{b} = \overrightarrow{b} \times \overrightarrow{c} = \overrightarrow{c} \times \overrightarrow{a} $
So taking the first part with cross product of $\overrightarrow{a}$ and $\overrightarrow{b}$ respectively
$$
\left\{\begin{array}{test}
\overrightarrow{a} \times ( \overrightarrow{a} + \overrightarrow{b} + \overrightarrow{c} ) = 0 \\
\overrightarrow{b} \times ( \overrightarrow{a} + \overrightarrow{b} + \overrightarrow{c} ) = 0
\end{array}\right\} \Rightarrow \overrightarrow{a} \times \overrightarrow{b} = \overrightarrow{b} \times \overrightarrow{c} = \overrightarrow{c} \times \overrightarrow{a}
$$
Is this way of thinking right, or the two equations have infinite solutions? I mean, no matter which vector we multiply with $( \overrightarrow{a} + \overrightarrow{b} + \overrightarrow{c} )$ it's going to be zero anyway.
So is this a gap in our reasoning?

It looks from the comments that you did not understand the details
that are missing from your proof, so here is a start. 
$\overrightarrow{a} \times ( \overrightarrow{a} + \overrightarrow{b} + \overrightarrow{c} ) = \overrightarrow0$, hence 
$\overrightarrow{a} \times \overrightarrow{a} + \overrightarrow{a} \times \overrightarrow{b} + \overrightarrow{a} \times \overrightarrow{c}  = \overrightarrow0$.
Use that $\overrightarrow{a} \times \overrightarrow{a}=\overrightarrow0$, and that $\overrightarrow{a} \times \overrightarrow{c}=
-\overrightarrow{c} \times \overrightarrow{a}$ to obtain
$\overrightarrow{a} \times \overrightarrow{b} - \overrightarrow{c} \times \overrightarrow{a}  = \overrightarrow0$, and finally 
$\overrightarrow{a} \times \overrightarrow{b} = \overrightarrow{c} \times \overrightarrow{a} 
$.
Then do a similar argument at least once more. 
