You could also take another approach by disabling the profile validation in the flows. But this also disables the email check, so, yes …
I’ve read somewhere that you can implement your own validation checks but to be honest I couldn’t be bothered.
It shouldn’t be this hard just to disable some fields on a registration page.
BTW If you want to create a custom form action than take a look at org.keycloak.authentication.forms.RegistrationProfile which is the Profile validation of those 3 fields.
You can copy it and adapt it to the things you want. Just make sure that the PROVIDER_ID is not longer than 36 chars.
You can than copy the registration flow and add it as an execution step and delete the original one.
This is something I wanted to do between other things 
but didn’t have time yet to fully implement and test it.