You have only stated that you have tested channels 3 and 8.
Have you tested other channels?
Also I do not know that it is interference, that was a speculation based upon the information you gave and assumptions based on my own knowledge and experiences.
If you have further information on the channels that do, and do not work that would help narrow down the possibility's.
Also you say an access point and range extenders. Are they passive signal repeaters, wireless access points (with cabled connections to the network) or active wifi repeaters (eg WDS)?
Have you tested different channels for throughput?
Has anything changed recently (new building or road works in the area, new network devices, or software/firmware upgrades)?