H
Hawkmoon
If the separate cables do not derive from a single OCPD etc, and do not terminate at the same point of load, then it isn't a parallel supply. So what you're describing isn't a single paralleled circuit, it's 3 separate circuits and therefore those circuits will be subject to a grouping factor!! Unless of course they comply with conditions where the grouping factor can be ignored!!
In simplistic terms, if the paralleled cables start and finish from the same single points at each end of the circuit then they are classed as a single circuit and you don't apply a grouping factor to a single circuit. Other derating factors and the cables installation method do apply, and will if designed correctly, adequately look after the circuit cables....
Respect your opinion on this but I think we may have to agree to disagree - regards