This guide will help you configure your ARMA 3 difficulty settings in the server.
Go to the Configuration Files "server.cfg" and click - Text Editor.
Inside, scroll down and find the = // MISSIONS CYCLE.
From the // Missions Cycle line, change the difficulty from one of the following 4 options: Recruit, Regular, Veteran, or Custom.
The custom needs setting up though.
Put this parameter for example:
// MISSIONS CYCLE (see below)
class Missions
{
class Mission1
{
template="BMR_Insurgency_v1_44.Altis";
difficulty="veteran";
};
};