The amount of paint (base coat) for 3 gallons of ready to spray (RTS) will depend on the brand/paint system you are buying. Your jobber or painter will know. Don't skimp on the clear coat quality. Its what has the UV protection and provides the gloss. Plus be sure you apply enough while also not applying too much. Talk to your painter about an extra coat to allow for future polishing and paint corrections.
Can't stress enough the earlier comment to confirm and use the correct size of the metal flake the original formula used. If having a shop paint it, get 2 spray outs of the paint to be sprayed. Keep one of the spray outs for yourself to check the final product. (I have a long story about keeping one spray out yourself) 😗