I'm so sorry to hear you have this diagnosis. You've had some good advice from people here , and there is lots of information throughout the forum on MVD ( Both in the library section, and if you search on MVD/hearts). Sadly, it is endemic and the number 1 cause of death in the breed and is extremely widespread–half of all Cavaliers will have a heart murmur by around age 5, and almost 100% will have it by the time they are age 10.
As with humans, this is not a condition which can be cured, but at least it is a very well understood disease and there are a selection of medications that can be introduced one after another as needed, and which can add many months and sometimes even years to a dog's life. It is really important to see a cardiologist if you can, because by the time a dog is coughing, the condition is fairly well along and you would want an accurate murmur grade–vets are not particularly accurate at giving grades. To me, a grade 3 might seem a bit low if your fellow is already coughing and needing a diuretic. The coughing generally means the dog may have gone into congestive heart failure and the heart is no longer working efficiently enough to keep fluid from gathering in the lungs. Nonetheless, many dogs can go for quite a while with this condition at this stage.
I recently lost my nearly 12-year-old Cavalier to MVD. Just to give some context, she had a murmur for many years, and lived for about 9 months after she started medications. In the end, she was on about 5, including 2 diuretics.
Getting any extra weight off is so critical–every extra ounce means the heart has to work considerably harder to supply all the extra blood vessels that need to run through the extra fat tissue, and the heart will work less efficiently, causing the dog to struggle. I would work to get that weight off in consultation with your vet.
There is a lot of information on this condition and managing it, here: http://cavalierhealth.org/mitral_valve_disease.htm