First, you certainly have not contacted a Yahoo tech info because there is no person to person contacts with Yahoo. The person who told you that there were three other people using your account is a scammer and he/she can't do anything for you.
To help you, we must know if (yes or no):
When you created the account did you register an alternate email address or phone number to which you have access?
When you created the account, did you register a secret question which you remember?
P.S. We are CCM, we have no business connections with Yahoo