Best guess
Hi,
Not being familiar with the exact equipment status of your aircraft, this is a best guess answer:
If you have a page where the ANP for IRS(3) is given, as a piece of information that is separate from your "actual" ANP with GPS or other updates (DME/DME or VOR/DME), then that IRS only ANP will just be determined by a standard algorithm base on "average IRS performance". Reason for this - when you have only IRS info, you have no "real" position to compare IRS position with, so you(r) FMS can only work with assumptions, which is the standard IRS deviation algorithm (make a best guess).