In the dim and distant past I flew a 4 engine type where we frequently shut down 1 and occasionally 2 engines to save fuel. we also relit them mostly. If a rotor isn't turning when you have airspeed, throwing fuel and a spark in it doesn't help.
If an engine has failed then it did so for a reason. If the FADEC has an auto relight function that didn't work when the rotors were still spinning quickly then you have no chance of starting it from 0.
Having said that, when the last one fails do what you can and good luck to you.