User blog comment:Godisme/San'ou/@comment-1920550-20120726161308/@comment-4484409-20120801144127

Maybe the reason the captains weren't taken out by the top 3 Espada was because they worked together. Shunsui and Jushiro two of the more powerful captains who are used to fighting together are a considerable force to be reckoned with. Also, factoring in their experience and training in combat makes it easier to believe that they can fight on par with Espada. Toshiro had the element of surprise and the superior weapon so he could hold his own. Without Hachi's intervention, Soi-Fon and Marechiyo would have died or ran away. Aizen's power was already implied to be above captain's level, but he also had the Hogyoku too so that puts him over the top in the power section.