Not sure if you can fit a Pi 3 in the Dreamcast. That's why I went with the Pi Zero. Specifically the Pi Zero w, for wireless already built in.
At the time when I did this, the DreamPi software did not work with the integrated WiFi module. So I had to add the USB hub, for a WiFi dongle to be installed.
It was a fun project and fairly simple to do. All the voltages are there for these modern devices and the amp draw is minimal, so the Dreamcast can easily accommodate it.
Good luck, and post up pictures when you get it done.