CENTRIC 12363016 part is 11" rear drum and friction surface is 2.93 inches wide. Found this part# at Rockauto under 69 Coronet with 440 motor. It is very close to 3 inch wide.
CENTRIC 11103360 brake shoe for 3 inch wide 11" drum. for both front and rear. Found this application under 69 GTX.
There are much more part #s for 11X2.5 drum and shoes though.
So if you definitely need 3" wide drum and shoes, it is available.