Owner profile has settings available to it that secondary profiles don't. There are more differences like being able to restrict installation of apps, end the profile's sessions to put data at rest, and other such things.
This is expected. The "install available apps" feature only lists apps installed in Owner. This is intentional. GrapheneOS had previously extended that so you could use that feature to install apps installed in non-owner profiles, but the change was reverted.
To install the Google apps on your third profile, simply navigate to the "Apps" app in your app drawer and you'll find them there, ready to install.