Best to only seed the srand() once. I had problems because my PC was so fast the time was the same for all the srand calls.

Create a static var to tell if you have seeded the srand() and don't do it a second time
Code:
static int iCount=0;

if(iCount==0)
{
    iCount++;
    srand(time(NULL));
}
computer_choice = rand()%3; 
if (computer_choice == 0 && user_choice == 1)
There are better ways to do this but this will give you an easy solution.