DNSSEC with multiple domains at different registrars

I have 3 domains. Let’s call them ex-ample.com, exam.ple, and examp.le, each with subdomains www.
They are registered at Godaddy, Porkbun, & Moniker, respectively. examp.le is the main domain. The other 2 resolve back to it by 301 redirects.

I have my A records for all 3 domains listed correctly in my CF DNS dashboard, along with the CNAME for the www.examp.le.

Can I enable DNSSEC and have it work for all 3 or will it only work for examp.le and its subdomains?

Related, what are the specific instructions for enabling DNSSEC at Porkbun?

DNSSEC works independently for each domain. Cloudflare also treats each domain independently. So you would need to enable it on each domain here, then at your domain registrar.

In other words, create a separate Cloudflare site dashboard for each domain, each with its own DNS & DNSSEC?

You only have one Cloudflare account. But each domain has its own set of independent settings, such as DNS records.