ZeroSSL is weird, if you use their classic non-ACME interface then the free tier is indeed limited to 3 active certs which can't be wildcards, but if you use ACME then there's no limits and wildcards are allowed.

https://zerossl.com/documentation/acme/

> By using ZeroSSL's ACME feature, you will be able to generate an unlimited amount of 90-day SSL certificates at no charge, also supporting multi-domain certificates and wildcards.