Another alternate for day 3, could be Luzern-locarno (via Gotthard line)-Dommodosola-Visp-Zermat (free with Swiss Pass) is a nice trip.
Second that.
The Gotthardbahn is clearly worth it.
You could also visit the Swiss Museum of Transport in Lucerne, which has a small but nice collection about Swiss aviation history and a good exposition about the construction of the Gotthardbahn.
While, in my opinion, Interlaken/Jungfraujoch is overrated by many tourists.