You very likely do not need to replace the whole aeration system. Your problem is very likely just as esshup says - the compressor. Buy and replace with a similar sized(hp dual head) compressor for somewhere around $600-800 or even less depending on supplier. A 1/3-1/2Hp 4-6cfm compressor will easily operate all three of your diffuser units. If the pond is not over 17ft deep also consider a 3/4Hp rotary vane compressor that produces 10cfm and plenty of air to more strongly operate your 3 diffuser units. I run 3 units with 5 heads with a 1/4hp rotary. the rotary units are much easier to rebuild and will last longer than rocking piston compressors. providing the depth is less than 18ft.
The only thing that I am unsure of is whether a rotary vane compressor will fit inside the medium sized Vertex Cabinet. Also, I don't know if the cooling fan took a dump or if it is still working. I have replaced a number of compressors this year that look like someone poured aluminum sawdust inside them, and in every case the cooling fan was not working too.
One other thing is that the OP didn't mention what voltage the system is running. Both the cooling fan and compressor have to operate on the same voltage. FWIW, both the Air2 and Air3 use the same compressor (1/2 hp, 4.3 CFM), cooling fan and the cabinet is the same size as well.
