If your object is less than 10 000 polys you can try
the _importObj by Max Smirnov http://moi3d.com/wiki/MaxScriptArchive
But you must convert first your "3dm" file --->OBJ!

or the native SubD / Create ;)

For decimate OBJ itself : Zbrush has a terrific one : Decimation Master

Maybe the free online OnShape can also make that! ;)
https://www.onshape.com