How about the old Ulrich triple hopper? You'd have to redo the
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sides and gussets (grind off?) because it represents the AAR
alternate standard style of construction, but otherwise the body
shape appears pretty close.
They'd be foobies, but getting to an accurate IC offset triple is not an easy
path. The IC cars are a taller three-bay version of the ARA offset quad
familiar to HO scale modelers as the Athearn (and multiple knockoffs) quad, with
the earlier "stepped" taper. The following photos are of these cars after
rebuilding in the 1970s:
I remember seeing an in-process HO scale model of one of these cars out at the
first St. Louis prototype meet done from an MDC offset triple, but I don't
remember the modeler (it was either Dan Kohlberg or Mike Budde). The problem,
as always, with this model is the goofy deep side sill and shallow profile
hoppers, but it does have the stepped taper and, with Archer rivets now
available, doing the four side sheet reinforcements is now a lot easier.
Another route would be to convert an Athearn quad to a three bay hopper, but my
gut feeling is that this conversion would be too short in height.