hmm I understand problem behind the problem you have, but I think you failed at another point. First your trigger:
make a variable ("MortarCounter") which serves as a counter that repeatedly counts from .. well for example 0 to 7 and as soon as it reaches 8, it must be set to 0.
do this by making a trigger with a periodic event (i.e. 0.50 seconds), the first action something like "Set MortarCounter = Mortarcounter + 1" and then "IF Mortarcounter >= 8 THEN Set Mortarcounter = 0 ELSE Do Nothing".
this trigger will execute every 0.5 seconds, and theres this weird counter running in it. Why? Next line in the trigger:
IF MortarCounter = 0 THEN Order Mortar1 to "_Attack Once_" Center of(MortarTargetRegion1)
IF MortarCounter = 1 THEN Order Mortar2 to "_Attack Once_" Center of(MortarTargetRegion2)
IF MortarCounter = 3 THEN Order Mortar3 to "_Attack Once_" Center of(MortarTargetRegion3)
Get it? If the counter is 0, mortar1 will attack, 0.5 sec later, the counter is 1 and morter 2 will attack. 0.5 seconds later, the counter is 2, nothing will happen, 0.5 seconds later, the counter is 3 and the third mortar will attack! This would be a great solution, BUT:
"Attack Once" doesnt exist for targeting a point. Doh. And if we attack an area, the mortars will keep attacking the point neglecting the order you gave them.
Suggested solution: Make a spell, that looks like the mortar missile and which deals damage, (i.e. "Cluster Rockets" from the tinker, set cooldown for that spell to 0.10 and mana cost to 0 and replace the missile gfx). Now you can "Order MortarN to [Goblin-Tinker Cluster Rockets] [Center of Mortar1Region]".
I hope you got it, pm me on finishing the map, good luck,
klovadis