function validate(password) {
    return /^(?=.*?[A-Z])(?=.*?[a-z])(?=.*?[0-9])[A-Za-z0-9]{6,}$/.test(password);
}

Explanation:

^               // start of input 
(?=.*?[A-Z])    // Lookahead to make sure there is at least one upper case letter
(?=.*?[a-z])    // Lookahead to make sure there is at least one lower case letter
(?=.*?[0-9])    // Lookahead to make sure there is at least one number
[A-Za-z0-9]{6,} // Make sure there are at least 6 characters of [A-Za-z0-9]
$               // end of input