Hi there,
In any case I would not expect a lot of traffic to posts that are syndicated. Generally people use syndicated content to supplement what's already there. So if the idea is to get more traffic through these posts, I would suggest finding another way.
Having said that, my understanding is that syndication-source still works, but canonical will override it. If you have something that says they no longer recognize it I have never seen it. I've used syndication-source when I don't know the URL or where a CMS/API doesn't allow me to get the URL, as you can point syndication-source back to the root domain URL.
I've also seen sites just post something like "Originally published on http://example.com/page" without incurring any penalties.
To do it by the book, I'd suggest canonical first, syndication-source second, and a source link third. And in this case I'd suggest doing it by the book because syndicated content is generally not going to give you a lot of organic search benefit. Hopefully that helps.