Answer:
You keep them if you test 3 and all pass.
There are 18 good ones and 6 bad (making 24)
Chance of first passing is 18/24
If it passes, there are now one less good and one less total.
Chance of second passing is 17/23
If it passes, there are now one less good and one less total.
Chance of second passing is 16/22
Total is (16 x 17 x 18) / (22 x 23 x 24) = about 0.4 (0.403162055 is you really want to know).
So probability all are ok is 0.4
Thus probability you find at least one bad one is 0.6
Hope this helps you out!
