Naim Supernait 2 vs Supernait 3

there is a naim forum where you will have the best luck to find the answer to your query

Yes, the Naim forum is definitely the place, but I will say that the best thing you can do for any Naim component is to use the best outboard power supply they offer for the unit.  The differences are transformative. Really makes you aware of the effect a power supply has on a component. 

Based on folks on the Naim forum who have compared both, more people prefer the Supernait 3 over 2 while a few prefer the 2 over 3. The 3 was claimed to be slightly more clean and detailed than the 2. The 2 was said to have more weight and drive in the bass, more of the "typical" Naim sound.

There isn’t any consensus whether the Supernait 2 or 3 will sound better when used with the power supply (Hicap DR). Although there are few who prefer to use the Supernait without the power supply, there are many who run the Supernait with one.

The 3 has the built-in phonostage which will be useful to some people.