It is running a lot lower boost than it should for stage 3 at the moment, this maybe add to the problem? Was set up previously for the proper amount, so maybe running rich because of that?
Running lower boost doesn't make a difference - it'll only add more fuel relative to manifold pressure.
And at idle, there's no boost to think about anyway lol
Only way to tell if it is actually rich is to stick a lambda probe up its arse, and then the CO should be adjusted to the chip author's specification (but certainly below 3.5% at MOT time).
Idle mixture is mainly goverened by the coolant temperature, fuel pressure, and CO pot setting. If it's had the fuel pressure raised for "more power" (see if the tamperproof plug's still on the regulator) it'll be rich all over the shop. Best thing to do is get a basic setup done so that idle speed, boost pressure, fuel pressure and CO etc are all set properly.