Saikyou Onna Shishou-tachi ga Ikusei Houshin o Megutte Shuraba

From ComicKey: For the three strongest adventurers in the world, finding a man strong enough to be with them is a herculean task. He simply doesn’t exist. With that in mind, they decide to take on a perfect novice. The level-zero, unremarkable Cross Alacart and train him up to be their perfect match. Freshly expelled from Adventurers’ school, Cross sets his eyes on redemption; training hard and learning to believe in himself along the way.