For volume production:
Use a (I forget the actual name) end mill with a hole down the center. They are used in a lathe or in a mill to make a long thin post of the diameter of the hole. They cut only in the plunge direction. That should produce clean posts of the proper diameter on a mill. You might have to get a custom tool made.
Clamp the 3mm stock, cut the two posts on one side. Then change tools and mill that side. Flip the stock over and do the opposite side.
Ken
Kenneth Lerman
55 Main Street
Newtown, CT 06470