Head to the abandoned plantation building marked in the map in the Bayou to call a sitdown to your three underbosses. From hereon, you get to choose who will take control of a racket and a district. There will be sitdowns whenever you take over a new district. Here's the important part: there are 10 districts in New Bordeaux. Bayou Fantom is the largest and "free" territory; no one takes control of this district so that leaves us with 9 districts you can distribute to your underbosses.
The best way to ensure that balance is maintained, it's a good idea to equally distribute the districts to your three underbosses, so that means each one will take control of 3 districts each. Plan ahead to avoid any friction between the bosses. Assign the rackets of a district to the underboss who you'll give it to. This is because if you give a district to an underboss, all rackets in the district will be transferred to him. If you continuously ignore an underboss, he/she may rebel, forcing you to get rid of him/her via a Betrayal mission.
The upside of equally distributing the districts is that your underbosses will remain loyal to you and you'll also unlock a neat trophy for keeping them alive until the final mission. The downside is that you won't be able to reach their last two favors (requires max possible earn which can only be acquired by tipping the scales and giving a 4th district to him/her). The last two favors aren't really that great, unless you heavily rely on them. Besides, each of them had gone their way in helping you, what's the point of having them killed for some few favors?
The map has additional missions available. Meet up with Donovan to continue with the main story and continue going after Marcano or complete optional agenda and trafficking missions. Completing Agenda and Trafficking Missions will get you extra cash and increases the underboss’ loyalty. New agenda missions will become available when you give new districts to your underbosses.
Unlocks the following: