Let the digit in the number is xy

So number is 10x + y

Given, the digits of the the number is differ by 3

=> x - y = 3  ..............1

Again, the sum of a 2 digit number and number obtained by reversing the order of digits is 99

=> 10x + y + 10y  +x = 99

=> 11x + 11y = 99

=> 11(x + y) = 99

=> x + y = 99/11

=> x + y = 9  ..............2

Add equation 1 and 2, we get

     2x = 3 + 9

=> 2x = 12

=> x = 12/2

=> x = 6

From equation 2

     6 + y = 9

=> y = 9 - 6

=> y = 3

So, the number is 63