SMO 2018 Open Round 1 Question 17

Let $f_0(x)=\frac{x}{3 x+2}$ and for any integer $n\ge 1$, $f_n(x)=f_0(f_{n-1}(x))$. Assume that $f_{2018}(x)=\frac{x}{Ax+B}$. What is the value of 3 B-A?