My home network consists of older devices:

• RB951Ui-2HnD
  RB941-2nD
  RB941-2nD

CapsMan is configured and working.

I was planning to replace the central node with hap ax3.
I updated all devices to 7.13.3. But I can't configure CapsMan. Not in one
nor in the other direction.

Tell me, is it even possible?

It is but you need to install the right capsman version.
See wireless package for controlling legacy wifi devices.

See wireless package for controlling legacy wifi devices.

Beware that by installing legacy wireless package on hAP ax3 (to get legacy capsman) you're loosing wireless on hAP ax3 itself. See viewtopic.php?t=202578

Correct addition !

You're welcome! And tell me, am I right to understand that I can simply configure the wifi settings on the new device by analogy with my other devices and without CapsMan? And everything will work.
In other words, is CapsMan needed only to centrally manage the configuration?

I have one simple network with one wifi network.

Capsman is not needed per se.
You can always configure a device as a local AP using same SSID and security settings.
For most applications you will not even notice it is not centrally managed.

Thank you for the response. Now everything has become much clearer. In the end, I just configured the wifi on hap ax3 without CapsMan, and everything works fine.