I have also used my HANS with 3" and 2" HANS specific belts.
as others have said the 3" works, and is safe to use, but the 2" fits the device much better.
If i were buying a new harness i would only get the 2" HANS specific belts for HANS useage, i like them that much more since I am the one who buckles myself in the car, and can easily feel that the harness is positioned correctly.
I just picked up a new set for my formula ford that has 2" shoulder belts for the HANS, and the 2" lap belts. its the nicest harness i have used, and what i would likely put in all of my cars when the time comes to replace them.
1977 Crossle 32F
1999 Spec Miata