Use addEvent function. Here's my example of a triple attack:
local combat1 = createCombatObject()
setCombatParam(combat1, COMBAT_PARAM_TYPE, COMBAT_DEATHDAMAGE)
setCombatParam(combat1, COMBAT_PARAM_EFFECT, CONST_ME_MORTAREA)
setCombatParam(combat1, COMBAT_PARAM_DISTANCEEFFECT, 10)...