D40 exhaust with DPF filter

Have D40 Navara 2008 auto with DPF. What are my new exhaust options? Thinking 3 inch DPF back. Is this a good option if so what brand?

The turbocharger, dump pipe, CAT and DPF are all restrictions to the exhaust system so going larger from the back of the DPF to the rear of the car won't give you a great advantage. If youre expecting eye-popping performance improvements you'll wonder if you'd spent the money right.

You could remove the DPF (it's not exactly legal but many do it as it's very difficult for an EPA/Polivce officer/vehicle inspector to realise or detect. You could even remove the CAT although you'll end up with post-exhaust-open injection strokes dumping fuel into an exhaust for no reason at all (these are used to provide reaction material for the CAT). Removal of the DPF comes with extra issues, because the sensors in the DPF need to be managed (send a PM here to 'auggie', he sells replacement sensors).

Alternatively if it's just power you're after then the two biggest gains are from a decent chip (Unichip being the best of the bunch) or diesel gas. Many people have added one of these with a standard exhaust and experienced significant performance improvements (very noticeable).

Will the 3 inch exhaust improve the exhaust note?

You can get a different note in the exhaust, but remember it's passing the exhaust through the turbine. Have a listen to some Youtube vids of Navaras, there should be some there of a blue one at the 2011 Navara Muster that sounded pretty good.