Since my scope is only loading skid and its 275 psi @ 330 gpm. Is 3" Sch 80 is appropriate. I didn't see anywhere where if the welds on pipes need to be API 1104 other than it should comply ASME Boiler & PV code, section IX.
Mechanical design is not covered in the API measurement section. The piping can comply with ANSI B31.3 or 31.8, your choice, when it comes to welding and strenght. The dimensions for the piping is what the API MPMS get concerned with.