There are two candidates for the third spot here, depending on whether you send out the spam yourself, or hire someone else to do it.

1. Low response rates. You'll get better response rates from opt-in email.
2. Angering recipients. You'll never know how many potential customers you are turning off by spamming them.

If you send the spam yourself:

Your domain will be blocked by many ISPs.

If you hire someone else to do it:

You could end up in court. There are laws against spamming.