Satin glass or frosted glass?

Advantage of the film: you can change it. Patterns are also possible.
Disadvantage: Someone has to apply it. So far I only have experience with normal window sizes, I can't say if 3m is more difficult. It's definitely not rocket science.
Topic professionals: At the moment we are still living in a temporary apartment. Recently, sun protection films were applied to all windows here. The films themselves were applied neatly, but despite previous cleaning, many fingerprints and similar marks can be seen. They are probably trapped under the film forever. I expected more from professionals.
I can't tell you anything about the price and price difference. You can probably find that out yourself?

So far I have only experience with normal window sizes, cannot say whether 3m are then more difficult. It is definitely not rocket science.

Wrapping films generally cannot be rocket science, otherwise there would be no untrained alcoholics in the industry with satisfied customers.
However, handling such a large film surface "in one piece" is unwieldy and you don't practice that in everyday life. 80 x 300 cm film – if it were available in one piece, I fear the rolls are only about fifty-something cm wide – smoothing out without bubbles is not for beginners. But my recommendation is also quite clear: not to wrap fully.

is it so difficult or expensive to get an undivided 3m long fixed glass element?

I don't see any visual differences between frosted glass and milk glass. Am I overlooking something? What exactly is the difference besides the manufacturing process?

The glass at 3m length with only 1m width is still no problem in terms of wind load, but for the production of the frame profiles, the bending machines up to 6m are simply more expensive than those up to 2.40m and partly the profiles longer are not stock items. A tilt or tilt-and-turn sash in this size would also be no joy in terms of handling and mechanically more expensive. Apart from that, a single-piece slot like that also has an appearance to get used to – I would "incorporate" the floor transition in the division. Frosted glass is called that because of the milky color, it is not transparent and only weakly translucent; satin finish is a colorless roughening of the surface, as if you had gone over it with a belt sander or as if the surface were covered with tiny dew drops.

I prefer it satin-finished. Then I'll see what the price looks like. Good idea with the floor transition in the division. I could imagine a tripartition, with a window sash in the middle to open. Actually, the window should be fixed-glazed, so not openable. I like this look in a stairwell.

Take a slide. You don’t have to do it yourself either. There is surely an advertising printing company or something similar near you. They will do it for you for under €500...