Should I merge my 3 Zelgius or fodder them?

If you have a Brave Sword and Bold fighter(not really needed, but is super good for Black Luna fest) use +Atk-Spd
If not use I guess +Atk-Spd is still the best choice
Merge him, he's better that way, unless one of your units needs Panic Ploy or Fierce stance, use +Res-Spd for fodder IF needed

Depends on if you plan on using them. If Zelgius is a unit that you have no plans on using, then I'd fodder, or at least keep one.

If you want to use him, but hope to get one with better IV's, then I'd merge them all and wait for one with better IV's to pop up (whenever that'll happen).

If you want to use one now, then I personally would pick the +Atk -Spd Zelgius.

Unless a unit desperately needs panic ploy, merge them. He’s an absolute monster for arena scoring, and is definitely much better than his masked counterpart.

Definitely merge! Panic Ploy is good, but Zelgius is a fantastic unit to have!

Go with the +atk/-spd one as well, then try and get Bold Fighter and use the Quick Riposte seal on him. With that set up his speed isnt actually all that important since he'll be auto-doubling most of the time regardless of his or his opponents speed stat.

You could also go with +res/-spd if you want to fodder a legendary ike for that Warding Breath on top of Bold Fighter and Quick Riposte. Would make him a very solid multi tank with incredible kill pressure.

If you don't have access to Bold Fighter or the Quick Riposte seal though then the +spd/-atk one works fine as well. With Alondite his atk is still 49 even with a bane, which is respectable, and his spd jumps to 36, so with a spd seal and some merges your armored unit could hit 40 spd on his own. Not as good as BF+QR, but a good alternative if those abilities aren't option for you.

I use +atk/-spd and works like a charm, since I use him as a baiter.
Probably the only better IV set is +atk/-res.

He's the best red unit in the game. Don't fodder him!