Minute insects on the outside of a trio of vessels (6)
I believe the answer is:
thrips
'minute insects' is the definition.
I don't understand the remainder of the clue.
(Other definitions for thrips that I've seen before include "Sap-eating insects" , "Sap-sucking insect" , "Flyer" , "Small insects" , "Insects that feed on plant sap" .)