Can iKQ three single phase MCBs (one on each phase)be used on a three phase supply or would it have to be a three phase MCB?

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Yes it is possible to use single phase MCB's in the three phase distribution board, however, this is not good practice for the following reasons:

Damage to Motors - Motor loads or circuits that are critical to have three phase voltage may be damaged by the loss of one phase. This is often called "Single Phasing".
Danger to personnel - Someone working on the installation may assume that the fault on a three phase load has disconnected all three phases, but two are still connected
Danger to equipment - The fault was caused by a problem with the installation, therefore it would be good to disconnect the load from the three phase device

However using 3 x single pole devices would be similar to the protection offered by a fuse and fuse carrier.