First, we know Zoro will never lose Wado Ichimonji.
In chapter 955 we see that Zoro accepted Enma in exchange for returning Shusui.
In chapter 954, it was introduced Ame no Habakiri. Being one of the 21 O Wazamono swords, Ame no Habakiri is an extremely powerful sword on par with Enma. According to Hitetsu, it can cut even the “heavens”.
Once wielded by Kozuki Oden, alongside his other blade, Enma, it was bequeathed to his son, Kozuki Momonosuke. Since Momonosuke is currently such a weakling (in conjunction with his idolization of Zoro as a swordsman => there’s a reason why Momonosuke wants to be trained by Zoro and blindly followed the “sunacchi!” battle cry Zoro probably mentioned in passing/half heartedly) he cannot even touch Ame no Habakiri. Most cursed swords cannot be touched by weaklings (or they get possessed) and Oda appears to follow the stereotype that the stronger the curse, the stronger the sword.
Unless Hitetsu is going to magically upgrade Sandai Kitetsu (which is impossible in a real world bladesmithing mechanics in that the great majority of a blades quality is determined by its creation. Once the blade is completed, only minuscule changes can be made and primarily only maintenance), Zoro will probably take Ame no Habakiri too.
So that leaves Zoro with three swords of the second tier (great grade). Some may be thinking, wouldn’t Zoro get three top grade swords? The key aspect is the black blade upgrade. Zoro will make them into the highest grade through the magical blackening. I would assume the information comes after the war(s) finish, but Hitetsu’s comments indicates he knows the exact mechanics of this “sword awakening”.
It may take several arcs, but I fully expect Zoro to have all three become black blades and this upgrades them to become supreme grade swords.
*Theory by Seiryu