If my understanding is correct, the explanation is as follows:

When P1 arrives, it executes first and has a wait of 0. When P2 arrives at the same time, P1 is executing so it must wait for the quantum to complete which is 4. When P3 arrives which is also at the same time, it must wait for P1 to complete its quantum AND for P2 to complete its one but P2 only has burst of 3 so doesn't use full quantum. Once P1 is finished its first quantum, it must wait for P2 and P3 to complete their quantums (or part thereof).

From this:

Quantum for P1 = 0 wait (first quantum)

Quantum for P2 = 4 wait (while P1 executes)

Quantum for P3 = 3 wait (while P2 executes)

Quantum for P4 = 10 wait (continues processing after 10 units i.e. after initial quantum plus those for P2 and P3).

Adding this up, you get 17 which is then divided by the number of processes.

I'm open to correction