The
Designer’s Commentary helps clarify what happens.
So in your example, you have a number of units that fight first. The Attack Bikes and Devastators (due to charging), plus the third unit in another combat. Your opponent has one unit that fights first (Krom).
Since you both have units that fight first, you alternate activations for the fight first units.
In your description, you activated the third unit in another combat first. At this point your opponent can either chose to activate Krom, or they can play the Counter Offensive stratagem to instead activate an eligible unit.
You then go back and forth until all of the fight first units have fought and then proceed to go onto the normal activation sequence. I hope that helps clarify things.