Sadly some less used but still popular aspect ratios are not supported. But unlike CS 1 and 2 forcing custom resolution does not break the game completly this time around. To use your own resolution you need to edit file "settings.xml" located in "appdata/local/trailsofcoldsteel3". You can simply right click it and choose: "edit"
Then change: <Resolution Width="xxx" Height="xxx" where xxx will be your native monitor resolution.

Expect slight issues with UI when you force unsupported resolution: in my case links are in wrong position and bars on BP counter are bit messed but it still shows numerical value.

For me that's a small price for being able to play in my native resolution.