Final answer:
The three main screen elements of the Now Platform User Interface are the Navigation bar, Header, and Workspace.
Step-by-step explanation:
The three main screen elements of the Now Platform User Interface are A) Navigation bar, B) Header, and D) Workspace.
The Navigation bar typically allows users to find and access different parts of the application quickly. The Header is usually at the top of the interface and may contain elements like search bars, user profile access, and system settings. Lastly, the Workspace is the main area where users interact with the applications, data, and services provided by the platform. While there might be a footer in some user interfaces, it is not considered one of the primary elements in the context of the Now Platform's User Interface.