Idk if theres a specific reason, but why are there three parties in the Roman Republic?
Historically there were only two, the Optimates and Populares (the Boni were the Optimates).
I have no idea about modding, but if someone could make a mod with two parties thatd be cool. :)

Also probably too hard, but are there any plans to update the election methods of the (Roman) republic. Consuls weren't elected by the Senate but by a council of citizens. Maybe the assembly selected could determine how elections were decided combined with different modifiers like popularity, political weight etc.?