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The rear opening of the Canon 2X version III extender now has a slightly smaller opening than the previous versions, and can not accept the front protrusion of any other Canon brand extender. You need to go with a non Canon extender to combine with the 2X III.
I believe that the long rumored Canon 200-400 f4 L IS lens also includes the reduced size opening, preventing you from mounting any Canon brand extender on it. So it is likely that the built in 1.4X will be the limit on that lens unless you use a non Canon extender.