Need regular expression to accept alphabets or numbers for first three places
Not all regular expressions are created equal. Can you tell us what language/context you're using REs in?
For example, many GNU libraries and tools would accept [:alnum:] as a valid character class.
So you could do something like:
egrep '^[[:alnum:]]{3}' some-file
That will output lines where the first 3 characters are alphanumeric.