If \( \mathbf{a}, \mathbf{b}, \mathbf{c} \) are three unit vectors such that
\[
\mathbf{a} + \mathbf{b} + \mathbf{c} = \mathbf{0}, \quad \mathbf{a} \cdot \mathbf{b} = \mathbf{b} \cdot \mathbf{c} = \mathbf{c} \cdot \mathbf{a}
\]
then the value of \( \mathbf{a} \cdot \mathbf{a} \) is