I have 3 circuits (microwave, disposal, and dishwasher) entering a metal box where the THHN wire will be spliced to Romex.
I guess that is a bit ambiguous.
These are three separate circuits as required by 2020 NEC, each on it's own breaker, black/white/green THHN runs for each.
I'll keep the different circuits' neutrals isolated, and thanks westward10 and Dave for pointing out the hazard of doing otherwise.