A 3-in-one cable is a special kind of connection cable used for pen displays to connect to a computer.
More here on the different ways a pen display can be connected to a computer: Connecting a pen display to a computer
A 3-in-1 cable typically looks like this:
The power end can work in different ways depending on which 3-in-1 cable you have
Older 3-in-1 cables may have a proprietary connection to the tablet instead of a regular USB-C connection.
Simplifies the physical design of the tablet. It minimizes the number of physical ports on the tablet. Instead of having an HDMI port, power port, and USB-A data port, the tablet can just have 1 USB-C port.
It also makes it easier to keep track of cables. You just have to have 3-in-1 cable instead of three separate cables.
Ideally you would connect your table to the computer with a single USB-C able. But this is not always possible.
USB-C ports on your computer may not supply enough power
USB-C ports on your computer may not support sending a display signal (aka DP alt mode)
Your computer may not have a USB-C port at all
So a 3-in-1 cable allows you to use older ports that many computers do have such as HDMI and USB-A and way to get enough power to the tablert.
