How to make custom keyboard input only capital letters?

I want to make the letters default to capital only, and ensure a minimum of 3 letters are entered but i cant work out the mask, currently it is this

[?Enter Customer’s Name;;;CS;]

Try this:

[?Enter Customer's Name;([A-Z]){3,}?;;]

spot on thanks @memo

Sorry, what was posted will allow an empty string. Change the ? to +

[?Enter Customer's Name;([A-Z ]){3,}+;;]

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spot on, thanks@memo