Hi
EdLondon
Welcome to Flyertalk and the EK Forum.
Can you tell us where you were informed that a 3 Class airframe would be operating EK358 from 26th September?
A check on the Emirates website to try and book F tickets for your travel date and random dates from 26/09 does not offer F so EK are not selling F. Checking on Expertflyer (the free version), the seat map only indicates a 2 Class 77W, again for a random date from 26/10 and your travel date.