If there are at least 3 models with Holy Triumvirate on your Army List, and they all have either Caelysian Pantheon or they all have Cult of Errahman, these models can all select spells as follows:
Example: I have 6 Priests, 3 Caelysions and 3 Errahman.
Are there at least 3 models with HT? Yes.
Do all of those 3 belong to the same Pantheon? Yes, I choose the 3 Caelysians.
And also yes when I choose the 3 Errahmans.
It is not clear to what the "all" refers to...all models with HT or just those 3+ from the first half of the condition. The rule can certainly also be interpreted as if you can have two independent Triumverates. This seems to me like RAI and RAW.
It says at least three models with... Its very clear you can't have it. Rule is written our well enough.
Only problem is disciples having to have Errahman to do what was intended with them.
#BringBackDiscipleFlails.... Maybe this takes off eventually and we can get a proper flails unit.