See Wikipedia and Mendix documentationfor help on regular expressions.
But ill try to help you a bit, please bear in mind that the first 3 are only alphabetical characters and do not include numbers.
I think that should do the trick.
In addition to Pieters answer, in general you want to make sure that there is nothing else in the string, so start with ^ and end with $ (those match string start and string end). for example:
^[0-9]{9}$
or
^\s*[0-9]{9}\s*$
if the number is allowed to be surrounded with whitespace.