What file format are you using? If you are using AWD and exporting using the AWD exporter for Max, the right way to solve this is to apply a “Normal” or “Edit Normals” (can’t remember the name right now) modifier. Either modify the normals yourself, or just use the default (smooth) ones.
If there is a modifier like this applied to the mesh, it will have “explicit normals” which the AWD exporter will pick up and include in the file. If not, it will use the face normals as vertex normals, and that could cause the faceting.
If you’re using the 3DS format, smoothing groups should be respected by the parser.
If you’re using any other format, try to weld once in Away3D (or Prefab) using either the approach that Fabrice suggested, or using the Weld class directly in your own code.