Three numbers!

Let $a$ , $b$ and $c$ be three distinct real numbers satisfying the conditions: $\begin{cases} 2a = ab + c \\ 2b = bc + a \\ 2c = ac + b \end{cases}$ Find all possible values of $a + b + c$ .

Clarification: If you have -1, -3 and 2 as solutions, write your answer as -3-12 starting from the smallest number to the largest in a string.