Headers made of 3" stainless?...or do you mean the exhaust piping?
If its thin stainless,an exhaust shop can bend it for a reasonable fee and some have a mandrel bender for smooth transition bends
If its heavy pipe,you'll be hard pressed to find a bender with big enough dies on it for 3" and strong enough to handle stainless without destroying the bender unless you take it to specialy pipe shop with the equipment to do so.
Stainless is MUCH stronger than mild steel and i don't know of any decent priced bender like the pro tools or the JD2's that'll handle 3" pipe/tube.
I see alot of electricians at nuclear power plants use a bender for that works pretty well for stainless conduit,but it always leaves the bend a lil flat and/or kinked and it a wide radius
Whats the wall thickness of the pipe?