Chronic Obstructive Pulmonary Disease (COPD) with Peripheral Artery Disease (PAD): This combination can lead to severe leg weakness/pain due to poor circulation, fatigue from decreased oxygenation, and heavy breathing due to lung disease. The coexistence of these conditions can exacerbate symptoms, making this a plausible primary diagnosis.

Heart Failure: Can cause fatigue, heavy breathing (due to pulmonary congestion), and leg weakness/pain (due to edema and decreased perfusion). The symptoms can be interconnected through the body's decreased ability to efficiently pump blood.
Diabetes with Neuropathy and Peripheral Artery Disease: Diabetes can lead to neuropathy, causing leg pain, and PAD, leading to leg weakness. Fatigue and heavy breathing can occur due to associated conditions like anemia or cardiac disease. The combination of these complications can result in the described symptoms.
Severe Anemia: Can cause fatigue, shortness of breath (heavy breathing), and leg weakness due to inadequate oxygen delivery to tissues. The severity of anemia can lead to these symptoms presenting together.

Pulmonary Embolism: Although it might not be the first consideration for chronic symptoms, a large or multiple pulmonary emboli can cause sudden onset of severe shortness of breath, fatigue, and if associated with deep vein thrombosis, could lead to leg pain and weakness. Missing this diagnosis can be fatal.
Guillain-Barré Syndrome: An autoimmune disorder that can cause rapid-onset muscle weakness, which might start in the legs and ascend. It can also lead to respiratory failure, necessitating close monitoring and prompt treatment. The potential for respiratory compromise makes it a critical diagnosis not to miss.
Spinal Cord Compression: Can cause leg weakness, pain, and if high enough, can affect breathing. It's a medical emergency that requires immediate attention to prevent permanent damage.

Mitochondrial Myopathies: A group of disorders that affect the mitochondria, leading to muscle weakness, fatigue, and shortness of breath. While rare, they can present with a combination of these symptoms and should be considered in the differential diagnosis, especially in younger patients or those with a family history.
Ehlers-Danlos Syndrome with Vascular Complications: A genetic disorder that can lead to fragile blood vessels, among other symptoms. Vascular complications can cause severe leg pain, weakness, and if there's associated respiratory or cardiac involvement, could lead to heavy breathing and fatigue. It's a rare condition but one that could explain a complex presentation of symptoms.