Please help with Swiss Train Routing

I will be flying into Zurich. My time in Switerland looks like this:
Day 1 - Fly into zurich, take the train to Lucerne and/or Interlaken
Day 2 - From Interlaken, do the jungfrau mountain excursion
Day 3 - leave from interlaken to florence, italy

I am having trouble with the following:
1. Should I buy a swiss pass of some kind or point to point train tickets.
2, It appears the Jungfrau tickets are for a separate rail system, so it is better to book separately. Are there any discounts?
3. What is the routing from Interlaken to Florence?
4. Since #3 involves multiple countries, how is the ticket booked?

Thanks. Appreciate any help you can provide. Going through multiple sites like rail europe and my switzerland.com, I am totally confused on how to book the train.