Welcome the forum Rog.
You don’t need “special” cables for any Naim amp, although for the regular pre-New Classic power amps, Naim do recommend at least 3.5m per channel of low capacitance and moderately high inductance speaker cable to maintain stability (Naim’s NACA5 is ideal here). More on that can be read here;
The Supernait 3 is rather more tolerant than that though, and while you still should stick to a combination of low capacitance and moderately high inductance in the cable you use, you can likely use shorter lengths of around 2m per channel without any problems.
More of an issue though is that running a high level sub feed straight off the amp terminals can be problematic. At best, it’ll just lower performance somewhat, but it can also create buzzing, humming, hot running, and even force a shutdown. Much better to connect the high level connection from the speaker terminal end, that way it’s not “seen” by the amp. Best of all, no need to spend loads of money. It’s just a reference signal so cheap and flexible is the order of the day. We used to use Chord Leyline at the factory for high level sub connection and it worked really well.