When they went from 3 to 4, they dropped direct x plugin support. Between 4 and 5, they changed the behavior of the routing and busses. Going from 3 to 5, even with the full versions, is a little perilous. Trying to open a nuendo session with an essentials version a couple revisions removed just isn't going to work.
You can exchange projects between Mac and PC what is important is the project compatibility between Steinberg Software.
The following knowledge base article has a chart with the information you need: