3. In an equilateral triangle $A B C$ with side length 1, let $\overrightarrow{B C}=\vec{a}, \overrightarrow{C A}=\vec{b}, \overrightarrow{A B}=\vec{c}, \vec{a} \cdot$ $\vec{b}+\vec{b} \cdot \vec{c}+\vec{c} \cdot \vec{a}$, the value is ( ).
A. $\frac{3}{2}$
B. $-\frac{3}{2}$
C. 0
D. 3
3. B. $\vec{a}$ and $\vec{b}$ have an angle of $120^{\circ}$ between them, $\vec{a} \cdot \vec{b}=|\vec{a}||\vec{b}| \cos 120^{\circ}=-\frac{1}{2}$.