The third limb is posture. Asan/Posture means a relaxed posture i.e. so restful, that there is no need to move the body at all. Do not confuse with the extreme Yoga postures that people do. True asana/posture is when one learns the grace of the body. The body is just like a guitar. It has to be rightly tuned; only then will the correct music arise out of it. If the very instrument is somehow not in right shape and order, then in no way can great harmony arise out of it? Patanjali says "STHIR SUKHAM ASANAM" -- the posture should be steady and should be very, very blissful, comfortable. So never try to distort the body, and never try to achieve postures that are uncomfortable. If you can be steady and comfortable in a chair, it is perfectly okay -- no need to try a lotus posture and force your body unnecessarily. A posture should be such that one can forget the body.
How to attain to this posture? How to attain this steadiness? Look at the comfort. If body is exactly in deep comfort, in deep rest, feeling good, that should be the criterion with which to judge.