Yes, set a test condition.
Code:
if ((y != 15) && (x > 100)) {
y *= 3;
}
At least insofar as the details asked. For other circumstances, you can adjust the tests, etc.
Or you could also have it such that if x == 100 or x == 101 you take action to multiply y at that time, but it all depends on whether or not x will always be at a specific value, or instead if also it can hit that value repeatedly.
As pan64 is saying, just using a flag:
Code:
int done = 0;
loop start
if ((x > 100) && (done == 0) {
y *= 3;
done = 1;
}
Perhaps more details, unless these rudimentary thoughts are enough to solve it for you.