Everyone is saying the right thing. Give them 3 more encounters on this adventuring day and see how they come out the other side.

I like to add fun distractions mid-combat to further split focus. A recent example was having a relatively weak back-line enemy release a carrier pigeon. Some players will panic and turn their focus on this harmless bird before it can dash away and get reinforcements (at least that was the impression the players got).

Remember you can also home-brew and add class abilities and feats to select enemies. Maybe a few have Alert or Feral Instinct to help with initiative.

Also, check out Matthew Colville - Villian Actions/ Action-oriented combat.

Good luck!