3 adults traveling this itinerary to see passion play mid September. Should we buy 3 country Eurail ticket? How would we get tickets to the play and one night in each of these places? Haven't purchased tickets yet, thinking of flying into Rome, then this itinerary and fly out of Munich.
Your question about the Oberammergau passion play is being discussed on this thread.
I'm afraid I can't help you with Florence to Venice to Luzern (to Zürich) but you can go from Zürich to Oberammergau via Munich with Europa-Spezial Schweiz tickets as low as €39 pP. These tickets are date and train specific; advance purchase (at least 3 days, up to 3 months, in advance) is required.
Later, all three of you can go from Oberammergau to Munich with one €28 Bayern-Ticket. Bayern-Tickets are always available and require no advance purchase.
So, of the four legs, all of one and most of the other can be done for about €48 pP. That's a whole lot less than two days of a railpass.
BTW, a lot of Luzern to Oberammergau connections would go through Austria, which would not be covered by a 3 country (Italy, Switzerland, Germany) pass.