https://wccftech.com/microsoft-surface-book-3-no-thunderbolt-3-support/

It’s 2020, and a lot of Windows 10 notebooks are equipped with a Thunderbolt 3 port to open up a lot of expansion possibilities for users. Unfortunately, Microsoft doesn’t believe Thunderbolt 3 support to be a valuable addition to a notebook computer because the company has omitted it from the Surface Book 3, unfortunately.

While the Surface Book 3 doesn’t come with a Thunderbolt 3 port, Microsoft has still incorporated a USB-C port, so while you’ll still be able to connect an external monitor, drives, and peripherals, connecting the notebook to an external GPU enclosure isn’t going to be possible. Also, data transfer speeds through a regular USB-C port is slower as compared to a Thunderbolt 3 port, so there’s also that trade-off too.