The BBC has resumed posting Doctor Who serials to their Doctor Who Classic and Doctor Who YouTube channels.
I just had a rewatch of Earthshock, and it's still absolutely stunning. We don't talk enough about how strong the first half is. It's a great sci fi quasi-horror type story with deadly mysterious androids killing victims left and right, and the second half continues the sense of unstoppable menace while kicking it up a notch. It mixes things up from the usual "base under seige" formula in ways that make the fight so much more desperate and chilling.
Earthshock:
https://www.youtube.com/watch?v=AZ-i3RPfgQE
The Three Doctors:
https://www.youtube.com/watch?v=JtyuG1dzCUo
If you're not already familiar with the basic story of Earthshock, and you want to first see backstory for context, I recommend watching Four to Doomsday first. Or maybe first Full Circle, then Four to Doomsday, then Earthshock.
Full Circle is a bit slow and weird, so skip it if you want to save time and it doesn't...