Hello Janus D
The Swiss Travel Pass is your all-you-can-eat ticket; it offers unlimited travel on the day of validity, without purchasing additional tickets (except for moutain excursions).
https://www.sbb.ch/en/tickets-offers/travelcards/area-validity-map.html?layers=ch.sbb.geltungsbereiche.mvp-sts
However, in my opinion, the Swiss Travel Pass Flex for 3 days is not very useful for the money you have to spend (CHF 93 / day); I personally would not purchase it but rather Saver Day Passes (from CHF 52 / day) or individual tickets, all depending on what your plans are. The Swiss Half Fare Card is also a great offer to save money, depening on the total amount you will spend on tickets. For a clear advice, we would have to know your complete itinerary.
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
https://www.sbb.ch/en/tickets-offers/tickets/guests-abroad/swiss-halffare-card.html
Furthermore, my I ask what is your intention of visiting Interlaken?
Interlaken is not a destination, it’s a travel hub. No Swiss would simply visit Interlaken yet would go there to connect to trains taking them to Grindelwald, Wengen, Mürren, or Meiringen.
Kind regards,
Roland