In the customize style tab for format->number, go to decimals->x+2 and enter:
if {table.field} in ["N12","N13"] then
2 else
if {table.field} in ["N14","N15","N16"] then
3 else
2//or whatever your default is for other values
Also make sure that the rounding is set for at least 3 decimals.
-LB