First of all, decide, whether you want to use a multishot or a bouncing attack type.
If you would like shots at multiple targets at once, you need the ability barrage. It has its own parameters to modify (for further information, there are plenty tutorials on this topic).
The other possibility is to use a bouncing attack. For this, you need to change the unit's attack type to bouncing attack (where you can change it to missile/melee and so on).
Set the number of targets total you want to hit.
Set the AoE for full damage -> it decides in case of a bouncing attack how far your attack bounces.
Change in Game Constants whether you want to allow a unt to be hit more than once by the same attack or whether not.
Side Note: Dunno if bounce + multishot work together.
Attachment is a picture of which values to change - the values I set there are just the ones from a hero of a map I'm working on currently

Feel free to use your own ones
EDIT: Pharaoh beat me to it
