If the combination can be in any order, you may need to list the alternatives explicitly, depending on what your regex flavour supports:
^([a-zA-Z]\d{3}|\d[a-zA-Z]\d{2}|\d{2}[a-zA-Z]\d|\d{3}[a-zA-Z])$
Please check below expression as per your requirement
(?=(?:.*\d){3})(?=(?:.*[a-zA-Z]){1})^[a-zA-Z\d]*$
Breakdown:
Look for at least 3 digits:
(?=(?:.*\d){3})
Look for at least 1 letters:
(?=(?:.*[a-zA-Z]){1})
Define what is allowed between the start and end:
^[a-zA-Z\d]*$
you can check example here.
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