The scenario "Last Flight".

Is there a way to play all scenarios in the Tribes of the East 3.1? I really like the mod that combines all campaigns into one game; I don't need to have 3 games on my computer.

I think there is, but, in my opinion, it's not very interesting to play through campaigns and scenarios in 3.1 when they were created for earlier versions.

It's much more interesting to play with mods, but it's also more difficult.

It depends on the mod. But in general, playing through 3.1 campaigns and scenarios from earlier versions is usually easier. For example, the third mission for Railag, in my opinion, and not just mine, is the most difficult in the fifth campaign. But with the "Nimble" skill, it will become much easier.
Although when I tried to play through the campaign in 3.1, but with new, very tough neutrals, it wasn't very easy.