I have made several 3" Star Downpipes. Best easiest way to do it is take a 2.5" 5 bolt flange and weld the 3" pipe to it. there is no value to having a custom 3" flange cut because it will warp very easily and the turbo outlet is still only 2.5".

You can buy a V-band adapter but in reality your still gonna hve to bolt and gasket it to the housing and you start getting very close tot he radiator hose and P/S pump with the 3" bend.

I do recommend a V band coupler right before the 90* that goes under the motor. Makes disassembly and maint that much easier.