Traveling to Morocco

For such a short time (which I would not advise), your only real choice is to fly. Check Skyscanner for flight options.

My two favorite Moroccan cities are Fes and Marrakesh, but they aren't terribly close together, and I don't think you have time to see both in 3 days. Casablanca is mostly modern.

I don't think you need a tour, but be aware that you may be bothered frequently (constantly?) by young men wanting to be your guide. That's not necessarily a bad idea, to avoid getting lost in the souk. But assume the guide is probably getting a commission on things you buy. Or you could pick up half- or full-day tours on arrival, just as you might in any other tourist destination. The Tourist Office could probably hook you up with a real (licensed) guide. In my experience, if you aren't accompanied by a guide, much of the teenage male population wants to be the solution to that (non-)problem, and it can be very annoying. Although my experience is decades old, what I've read since then suggests that this situation still exists to some degree. I'd welcome hearing more recent reports from others.

At the time of my visit, a great deal of the local signage was in French as well as Arabic, and French was very widely spoken. The young potential guides, not surprisingly, could state their case in multiple other languages. I don't think you need have more linguistic concerns than if you were traveling to a small non-touristy town in southern Italy or eastern Germany. However, I am atypically fond of being surrounded by foreign languages.

There are a lot better places to visit in Morocco than Tangier. Others have mentioned two great places. Rabat is another good place to visit and perhaps spend a day there. Not sure what you will do for three days in Tangier.

Yes, Rabat is nice and useful to know about if for some reason your only alternative is to fly into Casablanca.

To me, going to Tangier for three days would be a bit like going to Tijuana for three days.