One of DC's earliest horror comics hosts, Cain is everything Abel is not, as he is cruel and bad-natured. However, even with an attitude like that, he's been known to aid superheroes in the past when they needed magical help on cases. His true cruelty seems to be reserved for his own brother, but even there, it might be a situation where he is just playing out a cycle forced upon him (after all, there was one storyline where someone else tries to kill Abel and Cain goes nuts over it).
Cain and Able were prominently featured in a story in Alan Moore's Swamp thing, which led to them getting even more of a big role in Neil Gaiman's Sandman, where their homes turn out to exist in the Dreaming, so they get involved in all of the Dreaming plotlines.
Capullo drew a distinctive strip in his hair, so the middle figure is likely Jason Blood, the human host of Etrigan. In the aforementioned battle between Morgaine Le Fey and King Arthur ended with Merlin bonding Etrigan the Demon to a follower of Morgaine named Jason. The resulting bond forced Blood into opposition with Morgaine, and as a result, they became mortal enemies, something that has kept up throughout the years.
This is because that now that he is bonded with the Demon, Blood can't die of natural causes. However, he also has to transform into a demon that is bonded with him, so we imagine that there is a significant trade-off there between those two things. As an expert in the occult, he as helped out Earth's superheroes a number of times, even temporarily joining the Justice League once for a storyline! Not the Demon, just him himself due to his great knowledge of the occult (he knows a few spells, as well).
The silhouette for the final member of the obscured trio makes it that it is Solomon Grundy on the team. Solomon Grundy was a man resurrected within the "Slaughter Swamp" after being killed by robbers. Solomon Grundy has gone on to fight nearly every superhero there ever was, starting with the Golden Age Green Lantern.
As an interesting facet of Grundy's character, if you ever succeed in killed him, he would just resurrect in the Slaughter Swamp, but this time with an entirely different personality each time. Let's hope that he is currently on a pro-hero personality.
These immortals are all working together to deal with the Nth Metal crisis, and Snyder and Capullo do a great job explaining that this crisis is a whole lot different from past DC crises, as it threatens everyone, even people like Cain and Abel who lived in The Dreaming, so they are all involved in resolving this situation. Seeing so many disparate characters working together is a real blast. It would be interesting to see them show up post Dark Nights: Metal, perhaps as DC's response to the Illuminati? As what makes this alliance stand out is the fact that a good deal of them are technically "evil." That adds an interesting new dynamic to the group (heck, their headquarters is the home base of the Legion of Doom!).