Hi QAVION
That's a nasty one for this time of the morning !!
However, having consulted with my mates, re. power supplies etc., the best we can come up with is this:-
Operating in StBy power, the Capt's ND will be active as will the L FMC. The 3 IRUs will also be operative initially, powered from the APU Hot battery bus, so the IRS(3) indication is correct. HOWEVER, if you leave it in that condition for more than 5 minutes, the Centre IRU will automatically switch off and you should then be left with IRS(L) as the ND indication.
I don't have access to an aircraft or the sim until tomorrow but will give it a whirl then (if I remember
)