Unfortunately, the 3M A3 cartridge is no longer available and is replaced by the 3M Model B3 Replacement Cartridge.
The 3M Model A3 Replacement Cartridge is designed for use in the 3M Recreational Vehicle/Marine Filtration Systems WV-A3, WV-B2, and WV-B3. NSF Tested and Certified. Part number 5613416.
The Model A-3 should be changed every 1 year or 15,000 Gallons depending on water quality and usage.