like will 
's E Apply 
slow to everyone effected by the splash
or will 
splash damage cause to be applied my everyone hit by the splash or just the person initially attacked
EDIT: and what about 
will that apply to everyone hit by the extra bolts too?
Jamaree3 votesI think not, but I'm not fully sure, go test it.
Kouga3 votesFrozen Mallet only applies a slow to Auto Attacks, specifically.
Hydra's splash damage is not, itself, an Auto Attack. Instead, upon landing an Auto Attack (or applying On-Hit Effects such as through Gangplank's Q, Fiora's Ultimate, etc) it creates an AoE of damage. But the AoE itself is not an Auto Attack.
Same goes for the splash from Kayle's attacks.
However, Runaan's bolts specifically apply on-hit effects, so you should be able to slow 3 targets at once with Frozen Mallet.
arc951 votesTy