I notice that there are three parts/files to download and install World In Conflict.

Is it required to install all three parts or will the game be able to run on 1 part installed?

thanks in advance.

All 3 parts are needed (with the same file name pattern and in the same folder). GOG keeps the file limit below 4GB because it isn't supported on older file systems and/or for less disrupted downloads (= data to re-download) because of network issues on the user's end.