Hi, @fxholl 
To clarify, the endpoint is stable and supported. It’s only the authorization method that is changing. I can provide links to our documentation and resources, but you’ll likely need a developer to assist in incorporating your private app and form(s).
Resources:
- Migrate to a Private App
- Migrating from the API key to private apps examples (HubSpot Developers YouTube)
- Authentication Overview (for a Private App)
- Authorization Header documentation (HubSpot)
One question to clarify, you mentioned having three “HubSpot” forms. Are those forms you’ve created in HubSpot and embedded on your page? Or are they three non-HubSpot forms you’ve been using an integration to sync to your portal? I am not 100%, and I would rather not guess.
Thanks for the additional information! — Jaycee