I just checked under Scope. There is no random option in 2k3. There is an alternative, but it's complicated. You'd have to create the effect of randomness in every individual enemy troop.
* First, the skill you want to make, change the "Skill Type" to "Switch". Choose a switch that you'll dedicate for this spell.
* Now, in the enemy troop, you'll want a page condition for that switch being turned on.
* In the script below, set a variable to a random number. The range will be however enemies are in the troop. If there are three enemies, set it random from 1-3. If there's five enemies, set it random from 1-5. Basically we're assigning each enemy an imaginary number.
* Then, you'll want a conditional branch based on that variable. If the variable is 1, call a special effect and do to enemy #1 what you want the spell to do. If the variable is 2, do the same with enemy #2 instead. And so forth.
* At the end of the script, make sure to turn that switch back off.
Again, you'll have to do this for every troop in the game. Or at least, the troops that you can encounter while you have this skill.