Yes, Laroth is trying to usurp the precept of death from Arawn.
The Three Brothers are 3 mortals who managed to steal 3 gems of creation for Agares, just like Agares had stolen them from The One. The 3 gems stolen were the gems of Water, Air, and Death. The only brother whose name we know is Tuoni the Gatekeeper, the brother who has the Gem of Death embedded in his right hand.
There aren't a whole lot of details known about them. Most of what we know comes from Kael's D&D campaigns and aren't really cannon, but I believe that their existence is canonical. Tuoni was the main bad guy in one of Kael's campaigns. When the players met him, he used the Opalus Mortis (as the gem of death was called) to resurrect every enemy they had ever faced to fight them again. (The Baron Duin Halfmorn was one of the stronger enemies, but both times the team faced him they got extremely lucky rolls and killed him before he could land a single hit.) Tuoni was working with Tebryn Arbandi, who was casting the spells of winter. Tuoni wanted him to complete a ritual hat would kill everyone in the world, so that he could trap all their souls within the Opalus Mortis and use them to create a new world over which he would be a god. Auric Ulvin was supporting the rituals for most of the time, but did not want them completed since he intended to use the magical forces the ritual had gathered in order to ascend to become the God of Ice again. (I believe that in that campaign Auric was the resurrection of Mulcarn literally, not just a mage with the affinity for the sphere. I think that this might have ben the campaign where the land itself was formed from Mulcarn's corpse, and the sea of mists from his blood.) By saving the world, the team played right into Auric's hands and made him a god. The brother who held the Gem of Air was mentioned in that campaign, but they never actually met him. He was an excuse for the team's good allies to be powerless to help them and too busy to talk, as they were busy mounting armies to defeat a warlord who could but wave his hand and create an army of tornadoes to wipe out the armies and cities of men. The brother who held the Gem of Water was never found.
I tend to think that Ceridwen helped the three brothers steal the gems from Agares, against Agares wishes. I can't find it now, but I remember the pedia used to include a conversation between a pre-fall-Bhall (or was it a post resurrection Sucellus? I think it was Bhall and that the 3 brothers lived in the age of magic, but it might have been Sucellus in which case the brothers are around now.) and Agares in which Agares seems genuinely insulted that the good god would suggest he had anything to do with sharing the gems with mortals. Ceridwen has always liked sharing power with mortals, so as to corrupt them. I think that having access to an infinite well of the power of one sphere would cause a mortal to become completely imbalanced, so dominated by that one sphere that his free will is reduced to the level of a god. It would make Tuoni very dour and the brother who holds the gem of air extremely reckless (probably enough to make him die in some stupid accident). The sphere of water however would tend to calm a person and make him comfortable without using the power. That brother likely never used the gem enough to do serious damage to his psyche. Even if he did, the sphere would make him withdraw into himself and associate only with a few close friends instead of going out and harming others.
The brothers have a constellation named after them.
