Australian AMC 2008 - Senior Question 15

A function is said to be a toggle function on $(p,q,r)$ if $f(p)=q$, $f(q)=r$ and $f(r)=p$. The function $f(x)=ax^2+bx+c$ is a toggle function on $(1,2,3)$. What is the value of $c$?