The pattern goes like this:
one number, the next number is the first number doubled, then the third number is the second number + 1, and so on.
So, you see that 3 * 2 = 6, and then 6 + 1 = 7, and so on. This means that you have to multiply 63 by 2, and get 126, and then add 1 to get 127.
So, the next number in the pattern is 126.