Is Transport Fever 3 on GeForce NOW?

No, not at launch. A community manager at Urban Games confirmed in the Steam discussions that it would not be on GeForce NOW on release day and that the studio would look into it after launch. There is no date.

A twin-engine propeller airliner with a red tail climbing away from a runway into a bright sky — A propeller airliner lifting off the runway.

Who this affects most

Cloud streaming would have been the obvious way in for two groups the game otherwise leaves out: players on Intel Macs, which are not supported, and players whose processor lacks AVX2, which the game needs to start.

What you can do instead

Check your PC against the system requirements; the minimum graphics card is older than many people expect.
On console, it is on PS5 and Xbox Series X|S as a normal purchase; it is not part of Game Pass. See the Xbox page.