Lead-in to pod (3)
(Other definitions for pea that I've seen before include "Cause of a princess's insomnia?" , "Leguminous plant" , "12 in a pod" , "Pulse vegetable" , "Edible seed from pod" .)
(Other definitions for tri that I've seen before include "Three-point" , "Denotes three" , "Prefix denoting three" , "Prefix meaning three" , "Prefix indicating three" .)