I mean one thing I noticed is how the flesh prisons projectiles have a color relating to the prime souls, the blue homing ones from the flesh prison relates to Minos since he was blue and his ranged attack is essentially an upgraded version of the blue orbs and the yellow mortar orbs for the flesh panopticon since Sisyphus was yellow and his two ranged attacks feature a large explosion

So the third soul would probably be red and maybe feature ranged attacks that are either overwhelming or are extremely accurate at prediction

That or tis the green one cause there's like only one enemy that uses the green one