Wiring 3 smoke alarms and extractor fans

[ban-all-sheds]

I must pick on the point you have made ie "Can you multiply 3 x 3? Is the answer 3". Can you please be kind enough to explain how this equation works when attempting to calculate current consumption of a electrical device, ie my lighting circuit is a 6amp and assuming that I have 3 lights connected, by your logic if I now connect 9 lights I 'll need 18amps?. Please do explain as I'm very keen to learn from a professional.

Not the same at all. Your lighting circuit is 6A, but your 3 lights do not add up to 6A. If each light was a 460W device, then yes, adding another 6 would mean that you needed an 18A supply. But since they are (probably) not 460W items, you wouldn't.

The question you asked was "If the rating for a device is 3amps and say you have three of them. Can you use one 3amp spur box to connect all of them or do you need a separate box for each?".

If each one is 3A, then 3 of them total 9A. It's not that hard....