Losing compression in three cylinders

You need to do a wet and dry compression test. Do the dry test first, which is a normal compression test and record the readings, then do a wet test, that is with a small amount of engine oil in the cylinder, just a few squirts from an oil can will be enough. What this will do is it will seal the compression rings while you do the compression test, if the readings go up compared to the dry test any problem in those cylinders will be ring related and and if the compression's stay about the same or increase marginally, problems will be valve related. Do this test and report back the readings for further assistance.