My dad has third stay kidney disease and takes diuretics. but all of the sudden he's swollen in his knees, hands, legs....what might this mean?

Something changed: Either he had a big "dietary indiscretion" (salt-laden meal usually), or his kidney function has abruptly worsened, or possibly heart function has changed. All need examined and testing to see how to optimize back toward steady-state. Should see his dr (/specialist) for evaluation.

Fluid retention: He is retaining fluid from his kidney disease; it seems like he has a protein-losing kidney problem and his blood protein levels are lower - hence fluid retention in various body parts. Call his doctor. Needs check