If the 3 targets will live for 30 seconds it's best to mutilate each then DB

1DB on 3 targets is 300 potency where a full duration mutilate is 360
Shadow Fang is 2 hit combo of 150+440 making it 590 over 2 GCD's where two DB's on 3 targets is 600 potency over 2 GCD's and 2 Mutilates that will tick for full duration are worth 720 potency over 2GCD's

So if 3 targets that will live for 30 sec, you mutilate each, if less than 30 sec you DB spam and if 4 targets or more it's always DB spam.