While you should focus more on killing and interrupting, landing a snare style on someone nearby or while running to your next target is allways a good idea.
Who you should target depends a lot on the enemys setup obviously and whether your group has the upper hand in the CC/interrupt battle. Also keep en eye out for their peeler if they have one.
It also depends on if you use voicecom (discord etc) or not. If you do, the whole tank train can each be on a different target to CC/interrupt them, then collapse on one target to kill it once called for.
It's hard to create a flowchart for this sort of thing. Preferably you kill support first, casters second (assuming the other grp is locked down). If the enemy group positions themselves correctly and play well, it's rarely gonna go that way though.
Your job as a zerker MA is to try to break bubbles and land initial snare if possible on targets. You slammer is probably going to be peeling for your back line most of the time and be very busy while your skalds are fantastic at keeping targets locked down so they will mostly be doing their thing.
Its possible to throw 3 people at a caster on INC and straight up dust him, but in general it might be better to split focus and then call for a target that has been primed (no bubble, snared, potentially out of position) and allowing your melee train to collapse on them at once, so this will come on a case by case basis. I want to say this again that it is a case by case basis. Landing a solid mez and killing off people one by one is the best general strategy, but instantly getting your whole train Ichored because you wanted to "really mess up that caster" can be a fatal mistake and then those targets you didnt focus have a couple crucial seconds to stabilize, extend, alpha down one of your support, whatever.
If you are fighting against a zerg, then you just have to stack 3 guys on one target and hope to finish him before things get out of hand. When fighting a zerg its crucial to go for out-of-position targets that wont leave you open to 23452345 casters all targeting the huge troll.
You are also the forward "leader" calling for your assist train to pull back, which direction, call out those overextended and get regrouped based on support information.
The benefit of a melee train is how durable it is, use it to your advantage to grind down the enemy players instead of just hoping to alpha someone instantly every time.
I suggest calling a target and then saying primed or not. For instance on inc you say "On Celt Bard", you get him snared say "Celt Primed Assist" to let your other melee know its time to collapse. Two or three seconds of melee damage from a skald + savage + zerk with celerity is soul crushingly large and surprisingly hard to disrupt with all Midgard melee getting Det now.