Get top referrer with nested Iframes and cross domain

I’m trying to get the top referrer. My script is called in a (nested) Iframe over multiple domains.

I googled a lot, and I’m getting the idea that it’s not possible to do this.

For example:
Somesite -> iframe (domain A) -> iframe (domain B) -> my script(domain C)

No one any idea?

About the only way would be by using 3rd-party cookies.

What do you mean exactly with this?