You'll need a catalytic converter between the pre-cat O2 sensors and the post-cat sensor.
Either that, something called a lambda faker can be used. I've only seen this at the shop.alfisti site but it's probably available elsewhere.
The issue is Euro 3 engines monitor cat efficiency and not having something to tell the system that the post cat O2 sensor is sensing catalysed gas will bring on an error and limp home.
Either use a cat in there, or a lambda faker or have it remapped to delete the post cat O2 sensor monitoring.
I've heard a cat somewhere is your best bet as someone thinks TSs run better with "back pressure". I doubt it's back pressure but a cat may well regulate gas pressure fluctuations for better running. That's my take on what I read.