Actually, Fusion TP3 and TP4 both can open Inventor 2011 files. It means that TP3 and TP4 are compatible with open Inventor 2011 files. For your issue, can't open all Inventor 2011 files in your TP3? or is it just for specified file?
if just for the specified files, coudl you please send the file to me: email@example.com. Thus i will verify it and give you feedback.