Settings for Cross domain canonical

Hy there,

I read a lot, but still not 100% sure how to set up next situation. As example, I have 3 domains:

https://domain1.net/advertising (want to have this as primary!)
https://domain2.net/advertising
https://domain3.net/advertising

I have 3 domains, 3 pages (advertising) with 99% same content, just h tags changed, some words, etc (sites are for different regions in the same country).

So, my question, I added to last two domains (https://domain2.net/advertising & https://domain3.net/advertising) value for the canonical URL like this: https://domain1.net/advertising

But still confused, do i need also on this original page (https://domain1.net/advertising) to add canonical url the same like url https://domain1.net/advertising ? Or is it enough just to add can. url to domain2 and domain3?

Thanks, hope you understand my question, I want to avoid duplicate content or penalty from Google.

Thanks a lot,
Kany