Ordered serialization with semaphores

Consider the source code of processes $P1,P_2$ and $P_3$ given below. We require functions $f_1,f_2$and $f_3$ to execute serially, one after the other in this order.

Use two binary semaphores $S_1$ and $S_2$ to ensure this requirement by adding wait$(.)$ andsignal $(.)$ into the process source codes below.