If 2 out of 3 vectors $\overline{\mathrm{a}}, \overline{\mathrm{b}}, \overline{\mathrm{c}}$ are unit vectors, $\overline{\mathrm{a}}+\overline{\mathrm{b}}+\overline{\mathrm{c}}=\overline{0}$ and $2( \overline{\mathrm{a}}.\overline{\mathrm{b}} +\overline{ \mathrm{b}}.\overline{\mathrm{c}} + \overline{\mathrm{c}}.\overline{\mathrm{a}})+3= 0$ then the 3^rd vector is of length