Hi Jacob  
3" might be too big in terms of adequate back pressure.  There is a science behind it and others might be able to chime in with the scientific analogy around this.
I have a 2 & 1/2" (63mm) exhaust (no mufflers) on my 1998 300Tdi 130 for over 10 years.   
I can only ever hear the exhaust tone when the LH rear window is down and in a tunnel or drive beside a barrier. 
I like the sound actually.
I have had the car dyno tuned with this exhaust (along with other mods plus a pump tweak) and have plenty of power/torque.
				
			 
			
		 
			
				
			
			
				Andrew
1998 Landrover Defender 300Tdi 130 HCPU Expedition
1972 Peugeot 504 Sedan - Daily Driver
			
			
		 
	
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