Your regular expression should look like:
/^(?=.*\d)(?=.*[a-z])(?=.*[A-Z])[0-9a-zA-Z]{8,}$/
Here is an explanation:
/^
(?=.*\d) // should contain at least one digit
(?=.*[a-z]) // should contain at least one lower case
(?=.*[A-Z]) // should contain at least one upper case
[a-zA-Z0-9]{8,} // should contain at least 8 from the mentioned characters
$/