If you refer to the Mohlmann ...

If you refer to the Mohlmann index, your biomass in "aerator 3" is just fine and you are not using the full capacity of A1 and A2. Have a look at the color and the shape of the sludge flocs. They should be reddish brown and distinct, with streaks of water in between.

Mariusz Wuss has already given the advice to increase the recirculation rate to increase (!) the MLSS concentration in the first two.