Easy way to get 3 people is to go in with 3 people and wait til the random 4th person leaves since you didn't ready up, and then ready up. Hope that no one joins before the count down reaches 0. OR you could bring 4 people in, have everyone ready up and
as the countdown is at 1 have the 4th person leave so that the match starts with the 3 remaining people.
As for getting 2 people, I have no GD clue how people get on the 2 person leaderboards.
You need 3 peeps to ready up for a pub match, not 4. 2 is not enough to start the game, but if players leave, it can get down to 1 person left and still work fine.
I know you only need 3 people to start a game but if you try to start it with 3 people, it will take forever to countdown and by the time it gets to 0 someone joins. That's why it's easier to have 4 people join together and have one guy back out last second.