they're available on trademe - legality? Depends on how its interpreted technically it should be fine as it is 3 separate magazines that need to be changed and cant feed each other... Police may differ there though.
depending on your intended use for it - you may be better off fitted the magazine extensions to a couple of mags a just carry the spares - the extensions will make swapping easier and you wont have the extra width of the tri shape.
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