Stacking III extenders...

I used to have the Canon 1.4x II and 2x II extenders, as well as the Kenko PRO 1.4x teleconverter, and it was possible to stack them all with the Canon 100-400mm L. If I remember correctly the order was: camera + Kekno + Canon 1.4x + Canon 2X + lens. Now I am thinking of re-purchasing extenders, and I laid my eye on the III versions...

When the III extenders came out, however, I read that it was not possible to stack them as the II versions. Is it true? Is it not possible to stack the 2x III and the 1.4x III?

Is it possible to stack the 2x III and the 1.4x II?

If stacking is possible, do you stack them in the same order as I did with the II versions?

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Thanks, Imre