it’s the same thing, effectively
I strongly disagree. The first two are substantively the same, I agree. But the third is a wholly separate category. I see 3 basic categories we're talking about here: you choose to work together at the start; you know each other already; you're forced into working together by circumstances. The key difference between the 1st and the 3rd is that choice. "We have the same patron" is still a choice to work for that patron, and gives room for someone to say "nah, I'm not working with these people". When the circumstances themselves directly force you to work together, there's no ability to turn around and say "I'm going my own way". Being kidnapped and having brain slugs put in your head is one way. Everyone arriving in the same town at the time the town is unexpectedly invaded is another one I've been in as a player.
The other key thing about in media res is that you don't have that "inevitable round of introductions that feels like that time at the start of school when everyone had to stand up to say their name and one interesting fact about them". You're thrown into doing things before there's any chance for that. You get to know each other not beforehand, as in case 2, but as the adventure is going.
To be clear, I'm pointing to BG3 as an example that I've only very recently (the last two–four weeks) started, and which serves as a good well-known example of something that demonstrates a good example of something I already know works well. It's not a game that made me realise I completely new way of doing things. In media res will require players be cooperative enough to care to act, but it doesn't require they trust each other or know each other immediately. It definitely doesn't require pre-written specifically-designed characters.