If you have your reasons, then disregard the free advice...
Aerators are generally poor at adding oxygen to water. Roughly, 1.5% of the oxygen diffused is dissolved in the water for every foot of depth. When it comes to diffusers, deeper is usually better.
Also, since your diffusers are 3 feet off the bottom, your bottom is stratified and anaerobic.
Unless you have your reasons, it may be worth experimenting with the diffusers at the bottom of the deepest part of your pond. Note of caution, depending upon how stratified your bottom has become, harmful gasses could be released when you first begin true bottom aeration.
