In case of a Triple IRS Failure in flight (if possible) the NAV accuracy reverts to LOW
Now that's as good as understatement as they go. While some folks might call it LOW, more accurate description of it would be: NONE.
GPS recievers are still serviceable but their output is used to feed IR part of the GNADIRS only (in order to calculate hybrid IRS-GPS position). There's no bypass or override that can send GPS position to the FMGSes - see scheme in FCOM 1.34.15. So with triple IRS failure (assuming complete failure, so ATT is lost too) in flight you're down to hand flying on stby instruments, in direct law. VOR/DME/ADF/ILS are still operative, but what will ND look like is anyone's guess as you will have no HDG data to allign your compass rose with your heading.
But don't loose any sleep over it, triple IR fault is considered to be so unlikey by the Airboos that there's no procedure to deal with it, unlike single or dual failures. Also all of the dual IRS failures I've heard of, developed after the airplane was dispatched with one IRS inop.