I would think maybe one might be able to find in a laboratory supply catalog or maybe a large a hardware store a brass fitting/adapter with a 3mm threaded end and a "barb" hose end on the other end, or perhaps one could find a simple male-to-male threaded brass coupling that might serve as an adapter?
The problem is the thread though close to 3mm it isnt the standard pitch must be some oddball thread bing uses.
Lou