route tips from Milano to Brienz begin. of October

I am slightly confused what you are planning here.

Dropping a car somewhere at Varesse or Triano

1) Varesse - do you mean "Varese"? Not an obvious tourist destination
2) Triano - do you mean Tirano, or Torino (aka Turin)?
Where did you pick up this car? Why do you need it in the first place? Why not take a train (with stops) from Milan to Brienz, or take a car all the way?

then taking it in Lugano for a Swiss trip leg. Thought of 1 night in Milano=>1 night somewhere in Como=> a day around Como=> Bernina exress?

Lugano is not on the Bernina Express route. From Milan, you go by train to Tirano (passing lake Como). Then via the Bernina Express route to St Moritz and Chur. Then you need to go west across Switzerland to get to Brienz
4 days (3 nights) is enough time for this route, stopping off in Varenna (on lake Como) for 2 nights and St Moritz for 1 night.

What are you doing after Brienz?

meant Varese, mistyped:) taking a car at one country and leaving it in another costs a lot. Point is not to get from MIlano to Brienz, but to see as much as we can on our way.

Point is not to get from MIlano to Brienz, but to see as much as we can on our way.

Which is why I suggested the Bernina Pass train route: Milano - Lake Como - Tirano - St Moritz - Chur - Brienz.

taking a car at one country and leaving it in another costs a lot.

Yes, but driving to another country then returning it to the country you started from does not cost extra. That is why I asked about your plans before Varese and after Brienz, to try and get a picture of the complete trip.

no , flying out of Zurich