Other than price, what is the main difference between Canon, Nikon, and Sony for DSLR Cameras?

And what do you recommend for a beginner going into Photography 101?

Sony don't do DSLRs, I think they're still making their SLTs (fixed mirror), but they're largely concentrating on EVILs these days.

Pentax (actually the oldest SLR manufacturer in Japan) include weather-sealing, pentaprism viewfinders, and body motors on all of their current models, and are better cameras on paper than their Canon/Nikon equivalents.

Canon & Nikon have extensive collection of lenses and many 3rd party lenses are available.
Sony is better known for their mirrorless cameras.
DSLR cameras have much better battery life than mirrorless which is important for me. Mirroless has a compact body which makes lugging a camera around a lot easier.

If you're trying to choose between manufacturers, I would go with the brand your friends are using so you can try out different accessories like lenses and flashes. Plus, shared knowledge between friends is helpful in understanding how to use the kit.

Any manufacturer will be good for beginning photography. You'll find that an entry level DSLR or mirrorless body is capable of taking stunning images as those images are mainly due to the skills of the photographer (the reason you're taking the course).

I recommend Pentax.

Excellent products and value for the money,
plus almost any K-mount lens ever made can be used on newer bodies.