Do it yourself

Hi Everyone my aname is Al, I joined this group a while back and this is my first post. My problem is the three toilet bowls in our home have been backing up. I haved used a plunger and chemicals and the problem stops for a while then starts again. I had a plumber check the bowels, sewer line and anything else they could think of three times, but they don't find a problem. Any suggestions. Thanks in advance, Al