A 3 digit number is such that it's unit digit is equal to the product of the other two digits which are prime. Also, the difference between it's reverse and itself is 396.

What is the sum of the three digits?

The required number is 236 and the sum is 11.

It is given that the first two digits of the required number are prime numbers i.e. 2, 3, 5 or 7. Note that 1 is neither prime nor composite. Also, the third digit is the multiplication of the first two digits. Thus, first two digits must be either 2 or 3 i.e.

22, 23, 32 or 33 which means that there are four possible numbers - 224, 236,326 and 339.

Now, it is also given that - the difference between it's reverse and itself is 396.

Only 236 satisfies this condition. Hence, the sum of the three digits is 11.