AirWave Historical Data Retention maxes out at 1500 days for 'inactive client' data, 550 days for active client data (see AMP Setup -> General -> Historical Data Retention). So for a customer wanting to maintain 3 years of data, they should run either a daily/weekly/or monthly client session report (make sure to check the 'include data on all sessions') and export as either csv or pdf to email or scp/ftp to another server for storage. Duration of the report depends on number of clients daily. If customer is a hotspot/airport/store with a lot of unauthenticated clients, then it should be a daily report. If customer is an office environment, then a monthly report may work.
Another option is to run the report, and then generate a script to collect the data using the report APIs (documented in Home -> Documentation -> State and statistical APIs -> Latest Report API).