Because those three are acclaimed as hell and appeal to a ton of demographics. Have you seen the fanbases Phineas & Ferb and Adventure Time have? For god's sake, Adventure Time appeals to small children, stoners, guys in their 20s, guys in their 40s, teenage girls, etc.
Do you have proof of this?
And the show has managed to keep a consistently good quality through it's entire run while appealing to all those demographics.
The first season has a 9.2 on Metacritic and a lot of people say it's the worst.
Don't know where you got this info but the first season only has an 8.9 user score, and none of the seasons so far have a metascore. Phineas and Ferb did worse, with only a 7.1 user score.
Also, on the IMDB:
-Phineas and Ferb has a score of 8.0/10 from 2,214 ratings
-Adventure Time has a score of 9.1/10 from 1,834 ratings
-Futurama has a score of 9.0/10 from 41,960 ratings
The fact that Futurama gets a 9 from over 40000 ratings shows more acclaim/popularity to me than AT getting a 9.1 from only 2000 ratings
So what? What is that supposed to tell me?