import re
def fun(s):
    try:
        username, website = s.split("@")
        web, extension = website.split(".", 1)
    except ValueError:
        return False

    username_pattern = r'^[a-zA-Z0-9_-]+$'
    web_pattern = r'^[a-zA-Z0-9]+$'
    extension_pattern = r'^[a-z]+$'

    if re.match(username_pattern, username) and re.match(web_pattern, web) and re.match(extension_pattern, extension) and 1<= len(extension)<=3:
        return True
    return False