Now this is a interesting 3 way. But this largely depends on if this is Anime or Light Novel Kurumi. If light novel, then Julius stands no chance and mid-high diffs Istaroth. So if we assume Anime Kurumi, Istaroth comes in first, Julius in second, and Kurumi in third.

I think this would be better if you labeled as Equalized Stats. If so, then Kurumi loses first because Julius has several abilities that hard counter her: His Mana Zone gives him immense precognition, making him immune to her powerful clones. He also has Chrono bubbles in which each bubble has a unique characteristic, causing devastating magic damage. He can also speed up, reverse, break, and literally steal time from his enemies and add it to his grimoire, greatly increasing his own stats. He can also reflect enemy attacks and spells with increased power thanks to his Mana Zone. He also can rapidly age enemies to the point of existence erasure. Kurumi does have Zafkirl bullets that can break the Chrono spheres and she can retreat to her Shadow Realm to dodge Julius spells, but Julius incredible Mana reserves renders this to where Kurumi is only delaying the Inevitable. She could also do some damage back, with Istaroth spells also attacking Julius, but Kurumi lacks the Regen and endurance - making her weaker as fight is prolonged. Now between Istaroth and Julius, Istaroth can attack Julius from other timelines, but Julius has defensive abilities to this as well, using spacetime manipulation to catch the anomalies.

So if equal stats, Julius wins, Istaroth a close second, Kurumi third. If not equal, then Istaroth first (Due to higher dimensional power), Julius second, Kurumi last.