Can I buy the Navigo easy pass at terminal 3 using a machine? Or do I need to buy the Navigo easy pass from a teller?
Also can I load a mobilis zone 1-4 for Versailles onto the navigo easy?
Navigo Easy isn't sold at ticket vending machines yet. You can only buy Navigo Easy from tellers at ticket windows / ticket offices for the moment. The closest teller will be inside Roissypole which is the closest station to Terminal 3 for the RER B train to Paris.
You can load Navigo Jour day tickets onto Navigo Easy, but not Ticket Mobilis day tickets, which are the paper equivalent. The prices for zone coverage between Navigo day ticket and Ticket Mobilis are the same. I assume one day in the future, Ticket Mobilis will be phased out.