Difference between three 7.x branches

The module front page and the documentation do not mention at all the differences between the three 7.x branches (7.x-3.x-dev, 7.x-2.x-dev , 7.x-1.x-dev )

The three appear to be actively developed.
Are they all equally stable?
What features differentiate them?
Can we assume that 7.x-3.x-dev is the most feature-rich?