Simple!

It's funny that some of the information in the problem is extra,well let's say I could solve it without using the "difference between it's reverse and itself is 396" part.So here's how it goes:

Assume the number is $\overline{abc}$ and therefore according to the questions assumption, $c=ab$ where $a,b$ are primes.Now notice that since $c$ is lower than 9 then the only answers to $a,b$ are $(2,3) , (3,2)$ - I won't explain the reasoning since it's really really obvious! . Now without using the rest of the info,regardless of the number being $326$ or $236$ the sum of three digits is a constant value of $2+3+6=11$ .

Easy 75 points!

Let the number be $\overline{abc}$ . Thus, $a$ and $b$ are primes and $c=ab$ .

Now, $\overline{cba} - \overline{abc} = 396$

$100ab + 10b + a - 100a - 10b - ab = 396$

$99a(b-1) = 396$

$a(b-1)=4$

As $a$ and $b$ are digits, they are positive integers less than or equal to nine. Thus, the only solutions are ${a,b-1}={1,2,4}$ . But, as $a$ and $b$ are primes, the only solution is $a=2$ and $b=3$ .

Thus, the number is $236$ and the sum of digits is $\boxed{11}$ .

Note that in the second step, I took $\overline{cba}-\overline{abc}$ and not $\overline{abc}-\overline{cba}$ because as $c=ab$ , $\overline{cba}>\overline{abc}$ .