Prefect Cloud typically aims to support all features of the latest Prefect library versions, including those from Prefect 3.x. However, sometimes new features available in the open-source Prefect library may require some time before they are fully integrated or supported in Prefect Cloud.
The Prefect team usually ensures compatibility between the features of the library and Prefect Cloud, so most, if not all, features from the 3.x series should be operational in the cloud environment. For specific feature availability, it may be best to consult the latest Prefect Cloud release notes or announcements from Prefect. If you have a particular feature in mind, I can check for its availability.