[_0ffh]

Simplified, to survive one antibiotic, you need only mutate to resist that one antibiotic. To survive n antibiotics, you need to mutate to resist n antibiotics _at once_. Being vulnerable to just a single of them will kill you.

If the chance to get the right mutation to resist one antibiotic is, say 0.01 (1%). Then the chance to mutate to resist three antibiotics at once is 0.01^3, thats 1e-6 (0.0001%).

[chadthenderson]

You might need to simplify some more.

[undersuit]

I'm going to ship you a item. Luckily I have multiple copies of the item. I ship using UPS, Fedex, and USPS. You'll probably get three copies, but to get no copies of the item I'm shipping you something has to go ridiculously wrong in three different companies.