The leaves have a white powdery substance on then that is killing the leaves and one of the stems. They are definitly turning yellow. I have three plants and it is only affecting my favorite curvy branch plant.
Your plant most likely has powdery mildew, a common fungal disease. Separate the affected plant from the other two plants to prevent the fungus from spreading, and wash your hands and any tools after touching the diseased plant. Please see these articles for control options: