I think you need to change the #arrraymap call to:

{{#arraymap:{{{1|}}}|,|x|[[Category:x]]|\s|}}

...or change the template call to:

{{Categories|Categories=CAA,CAB,CAC}}

thank you Yaron, it works now.
I copied my template from this page https://www.mediawiki.org/wiki/Extension:Page_Forms/Page_Forms_and_templates, I guess this help page needs to be updated a tiny bit to make things easier to understand...

Well, there's nothing incorrect in that documentation, although it may be helpful to include a sample template call.

yeah, it does not contradict anything, it just would be helpful if there were a full example, say:

• here's the template
• here's the example of a call to that template...

this is what I meant.