True, but those 3 are probably stronger than the other 7 combined. 3 Vasto Lordes would make a pretty large difference, unlike the Adjuchas that have been fighting thus far. They could make quick work of the fodder captains, making less work for Aizen, Gin, and Tousen.
On the other hand, they might not even be needed. Honestly, Tousen could just use his Bankai in the middle of everyone, and all of the captains would be completely fucked. They wouldn't be able to attack, otherwise they would risk hitting eachother. So Tousen could just chill there in his Bankai while Aizen reduces the town to nothing.