Hi , the question of heavy copper  keeps poping up,although 3 oz is not
heavy i will try and help you, typicaly people deside how much copper they
need in the holes because of current distribution needed in the holes. you
can decrease the copper weight in the holes by adding more vias to the pad.
as for heavy copper we normaly start at 6 oz and go to 40 oz. these are
specialty boards your regular board shop can not make ( although not
difficult its a mind set thing). If I was to see the 3 oz  surface and 3 oz
hole board I would make it the following way.
 start with 1/4 oz copper material fr4 board and lay on 3 layers of 1.5 mil
dry film remove protective film each lamination. exposise and develop, plate
the full 3 oz so even with the surface add tin and etch , the 1/4 oz base
copper will etch quickly so the line resolution will be very good. I have
done 4/4 lines and spaces in 3 oz copper.

sorry the button plating does not work well because you can never get the
surface smooth,
not a advertisment but to only help people I have writen books on the
subject and they are available at megadawn.com, you can always e-mail me for
advice at [log in to unmask]
hope this helps.
robert Tarzwell

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