I have three teams, one in each country, but I am not happy with two of them. How can I resign from managing the teams?

You can delete them here:
https://ultrasoccer.de/teams/
With the "Löschen" button.

Though you would have to start over, if you rejoin the countries later at some point.
And you can`t recreate teams in the deleted countries for a few days.

come on, don`t give up... keep going!

@BamBaBam.....sorry but it`s hopeless with my teams and it`s not nice always being on the losing side. At least I am keeping one team in Germany.

@Ethan...thanks for the info.

You have to improve your tactics. Watch a few matches of the top teams

And you seem to still play with many of your starting players, who are probably not very good?
Most of the top teams replaced all players by now ... with "Weltfussballer" or at least very good "Bombe" players.
I`d guess that`s the biggest issue. :)