If I am not counting it wrong, there are currently 115 pets available for pet hunts (discounting starry pets, delve pets, cosmetic pets and pets released for Adana during the ongoing event).
In that case, the chance for a specific pet (and thus for a double pet) is about 0,86%, while the chance for a triple pet is 0,0075%, or 1 in 13225.
It is unknown, but not to rule out, that certain pets have a higher chance to appear during specific hours, days or months. If so, it has not been tracked down in the code so far, I think.