Closer to #3 than #2, so I voted #3. After a disastrous d20 Modern campaign a decade ago I learned that "you all meet in a bar" doesn't work as a trope (not literally, the PCs met for a job, but they were strangers).
Some years ago, my group would have several short games, most of which weren't D&D. (For whatever reason, the DMs that run the most stable games aim for D&D, Pathfinder, etc. Or Fate.) Fate ties characters together in the rules, so it's a #3. D&D's class systems aims toward #3 (you don't want to tick off the cleric, or wizard, or rogue, or fighter, because you need all of them) but doesn't tools to aim at that.
I've been in a large Exalted game (6+ players) where the PCs had no reason to be friends or do anything as a team, except that they were Exalted and lived in the same general area. We would have six hour sessions, the first five had us compete for DM time, and then the last we'd do a battle. Twice I tried allying with PCs, but both times the players moved out of town. (It wasn't me, I swear!) Eventually I started picking fights with nearby villains just so we'd stop the competition.
We had one DM whose last three games exploded because he made no effort to have PCs work together. I don't recall what the oldest of the three were, but the other two were Savage Worlds Deadlands and All Flesh Must Be Eaten. The last two had some passive-aggressive backstabbing which wasn't in the least bit cool.
One 4e game we're in somehow managed to get a group working together. I would put it at #2. It didn't do "you all meet in a bar" and we weren't strangers, but we have characters of incompatible alignments. Fortunately after a few sessions we've melded into a team, and it's not like anyone actually stuck to the alignment on their sheets anyway. There was no backstabbing.
I'm also running a 4e game (#3) and a friend is running a 13th Age game with some Fate added in (more #3 than #2).