Stable yes. even/flat? Not necessarily :-)
Cannot rock? Sure it can - you might need to exert some force on it in just the right place, but it could 'rock'
Consider a see-saw. There are three points of contact on the ground, and it is 'stable'. But put force on the high end of the see-saw, and it 'rocks'
Not as annoying as a four legged table can be, but still possible.