>> Day 1 i mean zurich- chur- tirano. ( overnight tirano or lugano?) to experience landwasser and brusio viaduct
You will need to make an early start from Zürich, as the Bernina Express leaves Chur at 8:32.
I would suggest spending the night in Lugano, which is a more scenic and interesting destination than Tirano. That means continuing on the Bernina Express bus from Tirano to Lugano on Day 1.
>> Day 2 tirano to zermatt overnight in zermat
I will assume you are travelling Lugano-Zermatt on this day. There are two good routes to consider:
1. Lugano-Bellinzona-Göschenen-Andermatt-Visp-Zermatt (6-7 hours)
The sector Andermatt-Zermatt is part of the Glacier Express route, so you have the option of choosing connections that include the Glacier Express (direct Andermatt-Zermatt), or travelling the same route on regional trains (change of train required at Visp). If you choose the Glacier Express, a reservation is compulsory at additional cost, and you might have longer to wait between connections at Göschenen and Andermatt.
2. Lugano-Giubiasco-Locarno-Domodossola-Brig-Zermatt (about 5.5 hours)
This route includes the scenic Centovalli Railway between Locarno and Domodossola. There is sometimes a small surcharge of CHF 1.50 or 1,50 Euro, which is collected on board by the conductor in Centovalli trains that have panoramic windows. Although this route travels partly through Italy, it is covered in full by the Swiss Travel Pass.
The timetable will also bring up a route from Lugano to Zermatt that goes through Milano, but I would avoid that one if you want your travel to be covered by the Swiss Travel Pass.
All these options will come up in the Swiss Timetable if you search for connections from Lugano to Zermatt, so you can choose the connections that suit you best regarding time of departure, preferred route, and acceptable time to wait between connections:
>> Day 3 Zermat to zurich is it pissible to do this or need to spend another day in st morits
If you want to see the Gornergrat, you will most likely need to spend a minimum of 2 nights in Zermatt, and travel to Zürich on Day 4.
You can travel from Zermatt to Zürich without going back to St Moritz, if that is what you are asking. For a more scenic route, put "Kandersteg" into the "via" field when you do the timetable search for connections from Zermatt to Zürich.
>> back to zurich (3 nights)
Do you have a particular reason for wanting to spend so much time in Zürich? It is the least interesting of your destinations in my opinion.
If I were you, I would spend only the last night in Zürich (and first night if necessary because of your arrival time in Zürich), and add extra nights to Zermatt and/or Lugano, which are both beautiful destinations with plenty of sightseeing opportunities.