73721 - Magnetic resonance (eg, proton) imaging, any joint of lower extremity; without contrast material

I have an MRI that was done as a "limited right lower extremity w/o contrast. They evaluated the knee, hip and ankle. Would 73721 be used with 3 units (for each joint evaluated) or does the code cover all joints in one? It's the 'any joint' that is throwing me off.