This is a great question! and one that I answer often! It is also one that is incredibly important to the health and longevity of your pup. My sister is on her second heart failure pup so I will give you the same exact advise that I gave her, (In fact I am also going to send her a little note to jump in and give her personal experience).
There is not a right or wrong answer here. But, here is what I think is the best answer. Heart failure in our dogs is most commonly associated with a heart murmur (I think that this is what you are referring to when you say 'level 3'?). We categorize a heart murmur on a scale of 1 to 6, six being the most severe. In may cases dogs have murmurs for years. Once identified they should be monitored closely with re-check exams at 3-6 month intervals, or sooner if there is any change in the clinical signs (like lethargy, coughing, increased water intake, reluctance to play or exercise, or weight loss, muscle wasting or anything else that seems different in attitude or behavior). If the murmur is progressing we monitor more closely or discuss further diagnostics or treatment options.
Ideally, I send every suspected heart failure pet to the cardiologist for an echocardiogram. These are incredibly helpful in understanding what the heart is doing and how it is affecting the patient. From the echo we discuss if, or when, it is time to start heart medications to help the heart work better. There are many helpful heart medications available to pets. Most are very affordable and have a very low complications or potential adverse side effects. An echo in my area of the Baltimore Washington area is about $600. Heart medications for my sisters dog is about $20 a month. Without an echo we are making an educated guess about what is going on in your dogs heart. This makes deciding a treatment option more difficult and potentially less beneficial.
Most of my heart patients do very well for years when on heart medications, monitored closely and kept on a good heart diet (ask your vet about these they are super beneficial) and kept on a calm, quiet, moderate exercise plan. My sisters first dog was initially diagnosed with a 5/6 murmur and lived 3 years after because she had the help of a cardiologist, medication, and a great prescription diet.
I am wholeheartedly in support of working up your pups case, getting as much information as you can gather from as many experts as possible and telling age to take a back seat. Proactive, diligent, and dedicated pet parents can lengthen the lives of their pets with both quality and quantity of good happy, healthy days.
Best of luck, and please let us know how your pups case goes. We are here to support you both!