Giant Cell Arteritis: This condition is a large vessel vasculitis that can cause headaches, joint pain (especially in the shoulders and hips), and vision changes (including double vision and loss of vision). The combination of these symptoms, particularly in an older adult, makes giant cell arteritis a leading consideration.

Rheumatoid Arthritis: While primarily known for causing joint pain and swelling, rheumatoid arthritis can also lead to headaches and, less commonly, vision problems due to associated inflammation.
Lupus: Systemic lupus erythematosus is an autoimmune disease that can cause a wide range of symptoms, including headaches, joint pain, and vision changes due to its potential to affect various parts of the body.
Migraines with Aura: Migraines can cause severe headaches and are sometimes accompanied by an aura, which may include visual disturbances. Joint pain is less commonly associated but can occur, especially if the patient has a comorbid condition like fibromyalgia.

Temporal Arteritis: Similar to giant cell arteritis but more localized, temporal arteritis can cause severe headaches and vision changes. Missing this diagnosis could lead to blindness.
Stroke or Transient Ischemic Attack (TIA): While less likely to cause all three symptoms simultaneously, a stroke or TIA can present with sudden onset of headache, vision changes, and sometimes joint pain if the brain areas controlling these functions are affected.
Multiple Sclerosis: An autoimmune disease affecting the brain and spinal cord, multiple sclerosis can cause a wide range of neurological symptoms, including vision changes, headaches, and sometimes joint pain due to spasticity or other secondary effects.

Sjögren's Syndrome: An autoimmune disorder that primarily affects the glands that produce tears and saliva, Sjögren's syndrome can also cause joint pain and, less commonly, neurological symptoms including headaches and vision changes.
Relapsing Polychondritis: A rare autoimmune disorder that affects cartilage, relapsing polychondritis can cause joint pain, headaches, and vision changes due to inflammation of the cartilage in the ears, nose, trachea, and eyes.
Vasculitis (other forms): Besides giant cell arteritis, other forms of vasculitis (like polyarteritis nodosa or granulomatosis with polyangiitis) can cause a combination of these symptoms, though they are less common and might present with additional systemic symptoms.