Thank you!
I'm preparing next release of game where will be some changes to gameplay, as different partisan's commanders, railroad diversion mission will be placed on real railroad on the map, rivers can be crossed only via bridges and so on. Time limit for missions will be also included, as other players asked for it.
To take down fortresses (garrisons) you need to send about 200-250 partisans on that mission. There are about 150 to 300 nazis in that fortresses.
So if you will remove all 3 garrisons you will get another victorious ending.