It's tough to pick just three functions as "all-in-one" devices come in many forms. However, here are three functions that are common and help define the concept:

1. Computing: This is the core function. Most all-in-one devices have a computer processor, RAM, storage, and an operating system. They can run software, browse the web, and handle basic productivity tasks.

2. Display: This is the visual interface. All-in-one devices typically have a built-in monitor, often with touch screen capabilities for interactive use.

3. Connectivity: This allows them to interact with the world. All-in-one devices usually have built-in WiFi, Bluetooth, and often include ports for wired connections like ethernet or USB.

Let me know if you'd like a more specific answer based on a particular type of all-in-one device (e.g., a printer, a home theater system, etc.)!