In order to boot from SSD you must follow a process only supported on the Pi 4:
- Install arm64 compatible operating system and get latest firmware on the Pi.
- Only Pi 4 is actually supported in this process, so it might fail if the Pi Foundation did not actually release the necessary firmware.
- How to change boot order on Pi 4
- Migrate over your ncp as described in the documentation.
Note, this entire process is only designed for the Pi 4. So, it might be possible, but you will see basically zero actual performance difference since you are running a Pi 3 on USB 2.0