What are the three main components of an operating system, and what does each do?

Shell (the user interface, CLI or GUI), kernel (bridges hardware and software, manages resources), and hardware (the physical electronics).

1. Shell - The user interface that allows users to request specific tasks from the computer. Requests can be made through CLI or GUI interfaces.

2. Kernel - Communicates between the hardware and software of a computer and manages how hardware resources are used to meet software requirements.

3. Hardware - The physical part of a computer including underlying electronics.

Go deeper:

• Operating system — kernel, shell and components (Wikipedia) — how the kernel mediates between hardware and software, with the shell as the user-facing layer.
• Cisco IOS (Wikipedia) — the network-device OS that puts these same three layers behind a CLI.