yes barrage is, its used as multi shot in alot of games, but dunt think itl work every 3rd shot even with a 3 sec cd (could be wrong) BTW siege engine has barrage.
BUT you could use Phonix fire (ability on phonix that burns ppls)
but set its dps to 0 and duration to .1 and cd to 3x ur attack rate
the only prob with this is it will fire even if your not attacking, and wont get faster as you get more agility (same with barrage if it worked)
but to get it to work with triggers your gonna have to creat a counter, that counts to 3 and resets, and at 3 it adds barrage to an invisible spellbook then after an attack removes it, id show you the code, but from the looks of this your not that advanced of a mapper, but if you PM me we can arrange a time to meet on USEAST so i could add this spell to your map for you