We have the following function..................................

void add_assign3(int *j) 
{
  *j = *j + 3; 
}

It's a method that has a return type defined as void.

You don't need to provide a return statement in such case. Alternate, you can do:

void add_assign3(int *j) 
{ 
  *j = *j + 3; 
  return;
}

If you want to pass back any value then you would return anything specific and accordingly setup the return type of your method.

Read for more details: Return from void functions in C++[^]