[filsteal]

I'm not sure if I quite understand what you're asking, but I'll try to give an answer.

Let's say something has a 10% chance of happening on the 1st try. If it fails on the first try, then it has a 15% chance of happening on the 2nd try. If it fails again, then it has a 19% chance on the 3rd try.

Suppose you want the probability that it happens on either the first, second, or third try. That's equal to 100% MINUS the probability that it FAILS on all three tries. So

1 - (.9 x .85 x .81)

is your answer, if I'm understanding your question right.