Ancalagon
Dusty Dragon
On roles with three players: You can cover most roles with three players - especially if the players work together to for a party - but chances are that one typical adventuring role will be lacking (or missing altogether. Backgrounds can help immensely there. But for me; that's also a plus in the "pro" column of three-player games; the party has a weakness. The chances of multiple players competing for the same niche is also reduced, and even when two characters are strong in the same role, that becomes one of the party's strength rather than "wasted resources".
I think all roles can be covered BUT not with simple characters. It also requires the 3 players all be willing to play along to optimize etc, so while it is possible, maybe it's not a good idea to force it? "Ok Joe, you have to play a paladin outlander so you can be the tank/healer/nature guy" "What?!? I hate paladins! " That's not great.
The main drawback IMO is that when one character is down, the party is amputated of 33% of their resources, which is huge!
Yup, which is why they need to be tough, or at least slippery.
Sorcerer is a bit more straightforward IMO. The hardest mechanics is spell slot / sorcery point conversion, but metamagic is relatively easy to grasp.
See I'm not sure that sorcerers (or warlock) are simpler, because you have to construct your spell list with a lot of thought if you want to be good at multiple things....