The Third Generation Demon Lords are the Demon Lords that have caused most trouble during the first arcs of the story. At the time of introduction to all the demon lords, Kazaream was not mentioned to be part of the Ten Great Demon Lords. However, considering that Leon Cromwell had killed him and taken his seat amongst the group about 300 years ago, it may be assumed he was most likely also part of the Third Generation (if not, the Second). Until the complete overhaul of the Ten Great Demon Lords into the Eight Star Demon Lords, they were the weakest of the generations, apart from Leon.
Kazaream was possibly the first known Demon Lord of the third Generation and during his time as one of the Ten Great Demon Lords, he was known as a well known schemer who continuously strove to increase his power within the faction by finding and recruiting potentially powerful allies under him. As seen where he assisted Clayman to become a demon lord,advocated Carrion's position among the ten and attempted to recruit Leon as a subordinate and offered to make him a Demon Lord.
Eventually Kazaream lost both his position and seat among the Demon Lord Council after he was defeated and killed by Leon Cromwell who then took over his position and territories as his own. Kazaream survived the encounter but was left in an extremely weakened state.
Carrion,Frey and Clayman are all part of the later generation of the thirds. With the three having become demon lords at around the same time and as such have known each other for the longest time and are shown to work together in pursuing their goals together. Due to the three being a younger generation of Demon Lords and as such have a lesser advantage in terms of personal powers,military strength and resources to the First and Second Generations of Demon Lords, are known to continuously strive to become more powerful by improving or enhancing their own abilities and recruit powerful individuals into their respective groups/nations in order to consolidate enough power in order to one day overpower the First and Second Generations of Demon Lords. As seen during their plot to create a puppet Demon Lord under their control to further give them an edge in the Ten Great Demon Lord Council.
This was due to the three of them being Self-Proclaimed Demon Lords instead of actual ones acknowledged by the Voice of the World and were thus extremely weaker than the first two generations by a wide margin.
Eventually after seeing the difference in power between them after seeing Rimuru Tempest overpowering and killing Clayman and seeing the power of a True Demon Lord, Carrion and Frey decided to work under Milim Nava and step down from the council.
Leon is the second youngest of the Third Generation of Demon Lords who obtained his position by fighting and killing Kazaream and taking his territory and seat among the Demon Lords after the latter attempted to make him his subordinate.
Uniquely from the other Demon Lords, Leon is in fact not a demon lord or anything of the sort but is instead a being on the similar level to a True Demon Lord, A True Hero acknowledged by the Voice of the World and was simply acknowledged and given the title of Demon Lord due to him defeating Kazaream. Making him both a Self-Proclaimed Demon Lord and a True Demon Lord due to the similar level of existence and power his title of True Hero possess.
Rimuru is the latest an youngest of both the Third Generation and the Octagram,having successfully Evolved into a True Demon Lord during the orchestrated battle between his Jura Tempest Federation and the Farmenas Kingdom by Clayman and successfully earned his position in the council after killing and eating Clayman.
Rimuru is noted to be the most powerful member of the Third Generation and the most influential due to his strongest friendships with all the other demons lords and him having consolidated power the fastest out of all the other demon lords and is the first person in the last few thousand years to successfully becoming a True Demon Lord instead of a self-proclaimed one like a majority of the third generations.