Triple Eight is a gas used by Owen Hind to get Dax to shapeshift in to a fox, in order to ensure he actually was a Cola. Because early in the series he could only shift when he felt threatened, angry or panicked, etc. Owen used Triple Eight to inflict extreme panic, so Dax shifted. He later apologised for its use.
It was mentioned again in Development class once, in B12, when Owen was talking to Mr. Eades. He tells Eades that Triple Eight had been necessary for him to shift intentionally so far.
When Owen tells Dax that Luke Reader had shown no signs of any Cola powers, Dax mentions Triple Eight again, wondering if Triple Eight would help figure out Luke's power. Owen replies that Triple Eight wouldn't work with Luke. This is because it gives the subject paranoia and panic, and didn't, as Dax assumed at the time, used to expose Cola powers.