In chapter 3 it is revealed that Rodrigo had sexually pursued Lucia. That should have been obvious anyway. Why else would he be against the marriage unless he wanted her for himself?
Renzo goes to a nearby town in search of a "dottore," not a doctor but a lawyer. He brings along some capons as payment. He has a hard time making himself understood. He asks: "What is the penalty for threatening a priest about a wedding?" The lawyer thinks that Renzo has been the one threatening (which he has, in a way),and lays out a plan of action. [Many verbatim quotations from legal books here.] But Renzo means Rodrigo's threat to the priest. When the lawyer figures this out, he has his servant women kick Renzo out, along with this chickens. There is no way he will cross the intimidating Rodrigo on behalf of a poor illiterate person like Renzo.
Meanwhile, at home, Lucia and Agnese, her mother, welcome in a Capuchin Friar, Frate Galdino, who has come to take them to the chiesa for the wedding. That's the church. Galdino tell a story I didn't understand very well about a wedding held long ago at the monastery. Then Lucia gives some money to Galdino (more than Agnese thinks is appropriate, but Lucia has her reasons) and they ask him to go in search of another priest, Cristoforo. This is the priest to whom Lucia had confessed (or confided) Rodrigo's pursuit of her.
So it's the classic melodrama plot. Evil rich guy stands in the way of happiness of young lovers. The narrative rhythm will follow a series of errands and encounters. Each potential helper will refuse or fail in some way to protect the weak. All spheres of life and society will come into play at some point, I predict: clergy, aristocracy, peasants, military men, etc... Of course the evil Spaniard don Rodrigo has to be the villain! Why did I never read this before?
The melodrama has the perfect structure for a narrative of the downtrodden being oppressed.