The "ACME Certificates" are free and unlimited. The 3 free "ZeroSSL Certificates" are old-fashioned manual certs: this is a strictly more generous offering than Let's Encrypt!

> If you have a server or other device that requires automatic issuance of certificates and supports the ACME protocol, you can use our free 90-day ACME certificates on all plans.

I think that refers to something else (manual non-acme certificates)? Many other pages says it's unlimited. E.g. https://zerossl.com/documentation/acme

I believe that is 3 hosts not total certs.

Zerossl is integrated with Caddy by default and there’s no indication from Caddy that you would only be able to renew the cert twice before needing to cough up some money.