Exhaust system for 190 D 2.5 ?

The exhaust system for 190 D 2.5 1987 has three (3) "noise pots". (The 190 D 2.0 has only 2). The third on the 2.5 that sits in the front what is that
for and what does it do and is it needed and can it be left out?
The exaust system for 190 D 2.5 comes in 3 part - front, middle and back section. Where can I buy this in Europe Germany online?
Regards Niels
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