You would likely need 2 pours of GlassCast 50 to be on the safe side so you don't get an exotherm of the resin.
If you use GlassCast 3 or not depends on if you plan on sanding the surface smooth and repolishing or not. If you are planning on sanding the surface anyway then just stick with the GlassCast 50 as any slight surface imperfections will be sanded out.
If you want to just pour a surface layer then walk away without further work, then the GlassCast 3 on the surface is the better option as it naturally lays smoother than GlassCast 50.