So this nerf wont handicap her in lane but it will make her not as much of a threat in a team fight as she is, my idea is that if she ults on one person it will do all of its dmg, but if she ults on 2 people it does half the dmg to each person, so on so forth. 
That nerf would completely gimp Katarina as a character.
If she hits three characters (1/3 damage to each person, as you suggest) she would end up losing 500 base damage + 167% AP scaling damage per target.
So that's a loss of a possible 1500 + 451% AP damage, overall. Think about that for a minute. Her ult would end up weaker on a per-target basis than several non-ultimate moves in the game, dealing only 250 + 83% AP damage to each of three targets...over the course of a 2.5 second channel.
Make it so assists don't reduce her cooldowns anymore, and she's fine.
Do you even league?
It'd have to be like 75%/75% then 50%/50%/50% split instead.
