There’s this site I rediscovered while pruning bookmarks. It is full of relatively simple models of great buildings (thus the well chosen domain name).
On OS X, at least since a few versions back, it is impossible to open and convert these files, except maybe with Cheetah. There’s a viewer that still works in Mavericks, but it doesn’t convert files.
From what I see .3dmf was based on QuickDraw, a technology that Apple has dropped a while ago.
Maybe these models have already been converted by someone and stored elsewhere.