Filters need regular maintenance and will clog and deactivate in time. With a 3-filter system you most likely have a 2-stage particulate filter and charcoal filter. You don't mention what brand of filter you have, but all of them are designed to be maintained by the consumer. Instructions for changing filters can usually be found on the replacement filter package. You should be able to identify the brand and vendors of replacement filters from the equipment manuals, or by looking at the faucet or filters under the sink. Like inkjet printers, the manufacturers make their profits from replacement filters, not the original equipment. Don't be surprised if the filters are expensive. If you can find the brand name, post back, perhaps we can help.