Whiecogie50 Zurich to Chur shows price 104.00, Chur to St. Moritz shows price 176.00. Couldn’t find price St. Moritz to Tirano.
It is not because you are taking three trains that you need three tickets. Tickets here are not for a train, they are for a route, and you can buy a through ticket between any two stations (or even bus stops) in the country. You are overthinking this a bit.
So the way to go about. this is to go to sbb-tickets.cc and look up “Zurich” to “Tirano” for your travel day.
You will be offered different types of tickets. Pick what suits you. There is no need to split this up.
You will probably be offered a Saver Day Pass as one of the options.