Try this as a first resolution, as the game will try to auto detect how many cores you have available, but this can be manually overwritten by setting launch parameters.
• Right click Arma 3 in steam library
• Select Properties
• Select SET LAUNCH OPTIONS
• In this field type / Copy and paste (Change the 8 to what ever applies to your system, if you don't know easiest way to see it under task manager, select performance (Win 10) and at the bottom of the extended view it says base information, sockets, cores, and L-processors)