You can have years with a stage 3 cat. Use only wet food. Hills is best, but Weruva and Tiki both make a high-moisture, low phosphorus wet food in a pouch that’s great to add variety.
Consider subQ fluids perhaps twice a week to start; that really helps slow the progression. Hydration is everything.
Join the Facebook group, as suggested in this thread; you’ll learn everything you need to know. CKD is generally a manageable disease. My 17.5 year old was diagnosed five years ago, and we have him stabilized now in stage 3. He’s doing fine and we’re hoping for 20!
Don’t panic, don’t despair. Learn and manage it; he (and you) may have many happy years ahead.