You do not take the rating of 3 screws as a safety margin. Each screw needs to be rated for what you are doing. I'm not an engineer but my good friend Lloyd Sikes of Airgun Lab is and when he told which 10-32 screws from McMaster Carr to use I did not ask twice.I'm positive if I sent him this link https://www.magnumairpower.com/pressure-testing.html I know what I would get for an answer!!Norm
There are several Crosman PCP models that use the same PCP tubing, similar fill adapters and similar valves. The Marauder Pistol and 1701P operate on 3000 PSI. They use the same tubing as the Discovery and Maximus.
As the screw head starts to lift it catches on recess in the valve and tensile strength starts to come into play. The more torque you have on the screw, the lower the remaining strength left to handle the pressure applied in shear.