are all human being's brain capacity is same..?

The actual physical sizes of people's brains are relatively similar unless there has been some kind of damage. However, the development of the connections and the brain activity can differ greatly. One example is that supposedly most people have three levels of consciousness and can use only one. However, it has been reported that Einstein had five and could use three simultaneously and constructively. There are probably other people who can use more than one and some who have more than three.

No. Some people have developmental issues that cause them to be less cognitively aware than others. Some people are also just normally slower than average. Of course, most people can work on improving their abilities through activities like reading and intellectual puzzles.

Not all brain capacities are the same. That is why there is such thing as IQ :)