Fair enough, it's requiring more skill to do it with knives.
It's just a shame you don't see throwing knives around that much - so yeah, if you got to do it in the same round, doing it on all three characters could/should be dropped, vice versa: if you should do it on all the characters or different players, it shouldn't have to be in the same round.
Overall the difficulty seems to be: all three characters in one round > 3 in one round > all characters across any number of matches.
I'd like to keep 'Hatshot' as the easier version of 'Hattrick' (i.e. doing it just once or across any number of games max 25 times or such) though, so maybe this could be 'all characters across any number of matches', idk.
I also support prop killing, maybe 'Cruel World' or something?