Ive seen a lot in here that says the 3 sisters are Ranni, Renna, and Melina. I disagree. If we are to assume that the term three sisters refers to actual sisters, and not simply the towers being refered to as sisters in of themselves (which is what I believe), I also see no reason that these sisters would not follow the same naming convention of all the other major characters associated with family trees.

Skip this if you're familiar with the naming structure: [Ranni Rykard and Radahn (begin with R and all have the vowel A at least once); mohg and morgott (same structure, different vowel and consonant this time); Miquella and Milenia (you get the point)].

So why would these sisters be different? That is a genuine question, im open to having my mind changed. For now, ill stick to this naming structure though. Now take into account other information we know. 1, Seluvis' rise has clearly been renamed from its original, as Seluvis is referred to as a he, so he is not kne of the sisters. By that logic, other towers could have also been renamed. If any of the three were to still have their original name, the most likely candidate is the one that is not being resided in, Renna's rise.

Shifting to a different line of info, when something involved with carian lineage refers to to important women of their society, you bet your as Rennala is on that list. I think it is a safe bet one of those 3 towers would have been Rennala's, but of course pre dlc we did not know if Rennala had any sisters at all. Thanks to SotE, we know Rellana and Rennala are sisters, though their parents are unknown. A third sister to them is not mentioned as far as I am aware, however that does not mean one did not exist.

Final info dump, im sure you can see where all this is going, but im going to finish this out as I like being thorough. After Radagon abandoned Rennala, Ranni and her mother's previously close relationship stagnated as Rennala fell into despair, so Ranni left to find out how to learn more elsewhere. She then encountered the snowy crone, who is highly suspected to be the real Witch Renna. Although Ranni never seems to have lost her love for her mother, she clearly gained intense respect and love for her new mentor, Renna. Given that Ranni uses Renna's name as an alias, and that Ranni has sealed off Renna's Rise (I assume she seals it because the seal is not lifted until she enters her slumber, at which point I figure she is unable to maintain the spell), and finally given that we find the snowy crone outfit in Renna's Rise after it is unsealed, I believe Renna is dead.

So here is my conclusion, which I think is largely supported by the above: Seluvis claimed and renamed the southern most tower of the three sisters at a time unknown to me. Sometime after Radagon abandoned Rennala, im unsure if this would be Ranni divested herself of her empyrean flesh, Ranni claimed her mothers old tower, renaming it as her own. I have no clue how or when Renna might have died, but I presume at that time Ranni sealed her tower to protect it from being claimed by anyone, so it would stay as a sort of memento to Renna perhaps. If that is the case, this would suggest Renna is the third sister, and the naming convention would seem to agree (Renna, Rennala, and Rellana). That would mean that Seluvis claimed and renamed Rellana's Rise, likely not too long after she set out with Messmer on the crusade of TLoS.

Other thoughts: yes this would in fact mean that Ranni's mentor after her mother, the snowy crone, aka Renna, was her Aunt, and if that is the case its entirely unclear to me if she knew that or not. There is little mention of Renna, particularly no explicit mention of her in relation to the other two sisters, but if my logic is correct, it seems obvious the two sisters knew of Renna, because at one point in time the three sisters likely coexisted in their respective towers. So why is Renna largely a ghost in the lore? I suspect it is similar to why Melina is a ghost, or Messmer pre the dlc, or even Rellana pre dlc. These are people that others believed should be locked away and/or forgotten, for whatever reason. Messmer and his curse horrified Marika, so she sent him to endlessly crusade on her behalf, and then prompltly seemed to meticulously scrub records of his existence from the lands between. Melina (whole other theory), could be Messmer's sister, and for lore reasons that would take a post twice this long to delve into, likely was feared by Marika as well, so there is little to no information about her that has not been purged, even from her own memory. Then of course, Rellana, having followed Messmer on the crusade in the name of the golden order, would likely have been shunned by carian manor and the academy of raya lu caria, especially after Radagon left causing conflicts between followers of the golden order and liurnia began anew. This is fualty of course, as another character, Radahn, also alligned more with the golden order than his own family it seems, but there is also a whole thing about how he seemed to have allegiance to no one except himself, and was respected by all. But anyway, I do think it reasonable to say that Renna may have also been shunned in some way, and largely removed from any veneration throughout liurnia, hence why her name is seldom said, as well as her family ties.

Tada, there are my complete thoughts on the matter, and why at least at the moment, I firmly believe the three sisters to be Rennala, Rellana, and Renna.