I think villas may have it correct.
I wonder if the old wives tale about selecting IR3 to avoid a direct law landing has been misunderstood.
I suspect that the way to achieve this is to switch the selection between IR1 and IR3 and back every 4 minutes throughout the smoke drill so that neither IR1 or IR3 exceed 5 minutes without power. If that is the case the smoke drill is busy enough without trying to remember to switch IR selections every 5 minutes.
It's just my guess and it may be that both IR1 and IR3 remain supplied in which case the only thing I can see against doing that is as Cough says, it may be ADIRU3 that's smoking. We'd need an engineer with a wiring diagram to advise if the IR switching isolates IR1 when selecting IR3 to Capt.