Thanks for the replies guys. It's been a busy week and I haven't given this much thought all week until today. I started setting up some of the encounters and have created some of the personalities for the 3 devil lieutenants. They are:
Scaramander (took the name from Adven. Tools): This devil is ambitious, enjoys power and like to lord it over others. His court consists of euphoric devils that utter prayers to him and act on his every whim. Part of his court includes a nightwalker and a larva assassin. He is very fond of flattery and he likes to have his power recognized.
Malphas: A decorated champion from the Blood War. He has many slaves, some demonic (even a Goristro slave) that is part of his court and bodyguards. Because he supports (perhaps forced to) an Outcast Devil, he is on dangerous ground but for him it has also provided more power and wealth than what he would have obtained otherwise.
Bist: This devil is incredibly cunning, aloof and generally keeps to his own court or to himself. In the recent past he has bound several Pit Fiends to several artifacts which he uses to power rituals or devices of his own creation. Perhaps the bonds that hold the Pit Fiends in place are weakening, perhaps not.
All 3 of these lieutenants share the same physcial location (that is a fortress) but each has his own area. They all follow Lamashtu (name stolen from Pathfinder) and Lamashtu is an Outcast Devil because 10,000 years ago an artifact from the Far Realm perverted his nature and altered his physical form. Now, his driving goal is open a permanent portal on the Prime to the Far Realm and bring the entity known as the 9 Nine Tongued Worm to the Prime. This does not align with the powers of Hell and therefore he was labelled an Outcast Devil.
The crux of the adventure is that Zamnaster (another name ripped off from somewhere

), who is the right hand man (devil) of Lamashtu and has aided the PCs before, has decided that now is the time to take down Lamashtu so he can have power for himself and remove his Outcast stigma that association with Lamashtu brings. Unfortunately, Zamnaster needs to do this secretly and the PCs through other means need Lamashtu killed as well. Thus, while Lamashtu and the PCs are not allies per se, their goals currently align. So Lamashtu has given the PCs some extra incentive to do this. The trick is, however, is that no one knows exactly where Lamashtu's abode is. To get an audience with Lamashtu, Zamnasters knows that at least 3 devils need to recoommend them to Lamashtu before they are given the location. Therefore, the PCs need to face each of these devils. I'd like to have a different dynamic in each one, some fighting, some skill challenges or a combination of the two so it doesn't feel like a straight forward dungeon crawl. They need to be contained, that is, no subquests or ongoing plots, sort of a 1 and done thing and then the devil recommends the PCs. I'm looking for something dynamic, perhaps with several phases befoere they win that devils confidence.
Hope that helps. Looking for more ideas!