Paukekea11
Your question is not specific enough, so we have to guesstimate….
For just those 3 train journeys you have mentioned, you would save approx. CHF 70.- thanks to a card costing you 120.-, that’s not the best deal.
But there’s no point just travelling to Interlaken and not leaving town for a couple of days, at least for most tourists. So if you should consider the (expensive, >200.- full fare) ride up to Jungfraujoch, then the half fare card would almost pay off just because of that single trip.
Also note that you would refer to the => “Swiss Half Fare Card” valid 1 month for 120.-, which is an offer only available for foreign visitors. The similar => “Half Fare Travelcard” (on SwissPass) however is a yearly renewable pass (actually a contract) which is intended for residents. Those two cards/passes are easily mixed up.