Cinelli LA84 x 3TTT stem troubles...

you should be able to use a round file to enlarge the inside diameter. I've done it with a 25.4 up to 26.0.

Hmm, that could be an idea

If the 3T stem is 25.8 mm it should work with bars that are 26.0 mm.
As with regard to shaving off metal thickness, if you must, I would suggest sand blasting with 120 or 250 μm at 2.5 to 4 bars pressure. I know, probably sound too specialised, but it's a nifty way to remove a relatively uniform layer of metal without introducing too much stress into the alloy. If you know a dental technician/ dental lab they have the equipment, and most lab use the sand blaster on daily basis.