D&D 5E 1-3 Encounters per Adventuring Day

[Asisreo]

I'd probably run them in initiative groups of 3 acting as a loose sub unit.

It's funny you say that, because I've just came out from a discussion where RAW all identical monsters have the same initiative. In fact, it's a complete houserule to separate like monsters since there isn't even a variant rule that allows this.