Of course it's available, from plumbing supply houses and even mass-market builders supply stores, possibly by special order.
You can probably also get broken lengths from scrap dealers, who buy it from junkies, who stole it from the last guy who failed to provide 24 hour armed security for it. ... which is why copper is being supplanted by plastics wherever possible, which is why it's less commonly stocked than in prior decades.
Thanks for the replies. The reason I asked is that the mechanical contractor told me that 3" is an "odd ball" size and although it is made, it is not so readily available. I have not tried Home Depot or Lowes. I will check with them if they stock it and what the delivery period would be if they do not.
Check with some local Civil Contractors. I have worked on a number of projects with CDC Contracting out of Concord. Ask them and they will point you to a supplier who has some - your contractor is saying this because they can't find it.
...and one other thing that I should have mentioned. It is type "L" (i.e. medium wall thickness) copper pipe that we seek. I will try to follow up with the suggested contractor. Thanks.
McMaster-Carr stocks it as part number 5175K61 , US$341.48 for a ten foot length. Plus shipping and whatever border hassles apply. Local sources should be more competitive.
