The loop will execute 4 times. Increments are made before the test of the next loop.
Code:
int main(void) {
   int i, x=0,n=0; 
   for( i = 0; i < 11; i += 3)
   { 
      x += i;++n;
   }

   printf("loop counter: %d\n",n);
   return 0;
}