You haven't specified which tale your question refers to, so I am assuming that you are talking about "The Miller's Tale" and the three large containers that Nicholas, the student, tells the carpenter that he needs to acquire to prevent their deaths because of the massive "flood" that Nicholas has discerned is going to come through his study of astrology. Of course, this of course is just a fabrication to try and engineer a situation to get rid of the carpenter so that Alison, the carpenter's young, beautiful wife and Nicholas can have an affair.
After the carpenter has acquired three large vessels or bottles for them to supposedly float in, he obeys the other instructions of Nicholas, packing them with provisions and also building ladders so that they can each reach their vessel without any problems:
And then with his own hands he carpentered
Three ladders, shafts and rungs by which to climb
Up to the kneading-tubs hung in the beams.
Next he provisioned them, both tubs and trough,
With bread and cheese, and good ale in a jug,
Quite sufficient to last them for a day.
Thus the scene is set and ready for the "flood" and the act of infidelity that Alison and Nicholas are going to commit whilst John the carpenter will be asleep in his tub.