does unity open C3D files ?

Generally, OBJ is ALWAYS possible with 3D applications, so, technically, I could say yes, C3D is supported. However, to what extent I am not sure of as I have not personally used C3D. As long as C3D can export to one of the following formats, then it is supported.

Here is a full list of Formats Unity support. Just hit Export on C3D, and check for them.

(PS: Scroll about half-way down the page until you see “3D Package Support”)