> "either of the"

- not consistent with there now being 3 of them (!)

Do we really need/want a full list here anyway, as this will only go out of date ?
I'd suggest maybe ...
""" ... \<Aggregator> instances, which specify statistical measures in the methods that calculate cube statistics, such as :meth:`~iris.cube.Cube.collapsed` and :meth:`~iris.cube.Cube.aggregated_by`. """