As goldenrivet pointed, ISIS is fed by STBY ports and probes. There are air data modules in STBY ports/probes to provide anemometric data to the 3rd ADR.
The confusion usually exists because in case of blockage in 1+2 the ISIS still works. The air data switching in position 3 recovers the problem and people associate the ISIS with ADR3, but in reality those 2 use the same tubes.