Validate email with regex having exactlly 3 letters in TLD

I want to validate an email in a function with an email that meets the following conditions:

A valid email address is of the form username@domain.tld, such that:

• The username consists of only English characters, numbers,
  underscores and periods.
• The domain consists of only English characters or numbers.
• tld is a three-letter word of English characters.
• English characters can be lowercase or uppercase.

I have written the following code but I don’t know why it doesn’t work?

import re
def validate_email(email):
   regex = re.compile(r'^([a-zA-Z0-9_.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z])$')
   if re.fullmatch(regex, email):
        return True
   else:
        return False

I want the domain to be exactly three letters like com tld edu …

>Solution :

you have added this \.([a-zA-Z]){3}$

def validate_email(email):
   regex = re.compile(r'^([a-zA-Z0-9_.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z]{3})$')
   if re.fullmatch(regex, email):
        return True
   else:
        return False

now is working