My understanding is that the said form or forms should be the same (the looks, the events raised by user interactions) in AutoCAD, ZwCAD and BricsCAD. So, the form(s) should be in its own project (DLL) so that it can be shared among the 3 products. However, in order to share the View (the form), you should completely separate the business process/data from the view, so that the view only plays the role of presenting data and taking user interaction (and passing the interaction to business layer).
You may want to look into the MVC pattern. That is, the View(form) project only includes Views that can be shared, maybe the data Model (if the 3 products can share the same data model). Most likely, the Controller will be different for each products. In the shared View project, you can define a IController interface, and then implement 3 different Controllers, and so on.