In my flight plan I put at waypoint 31 (walky) a new altitude of FL200. How does PF3 work, he will bring me down so I pass this waypoint at FL200 or it will tell me to go down to FL200 when I have past the waypoint?
The altitudes you enter on the flight plan page are crossing altitudes, so if you enter FL200 for WALKY you will be cleared to descend to that altitude before you reach the waypoint.
However, you may have a problem with the descent profile in this case. If you change a waypoint altitude you should check the altitudes of nearby waypoints to see if they need to be adjusted as well. Your altitude at MENRO (17 miles before WALKY) is unchanged at FL380, and I'm not sure if PF3 will start you down in time for a comfortable descent. To descend from FL380 to FL200 you will need 50-60 miles or so. You can hover your cursor over the waypoint names on the flight plan page to check the distances to adjacent waypoints. If I were setting this up I would probably want to be at FL310 at TELCO and FL250 at MENRO. If you try the flight without changing those altitudes and PF3 starts you down too late, you will need to be requesting lower.