There are several diseases that affect the leaves of hydrangeas, and your plants may be experiencing one of the. It is common to see them show up late in the season. There are both fungal and bacterial diseases that cause spots on hydrangea leaves and they vary in appearance on the different species of hydrangea making it difficult to confirm a diagnosis from pictures alone. Below are some articles discussing the different diseases of hydrangea leaves. Many are cosmetic and do not affect the overall health of the plant.
Fungal leaf diseases: http://hortsense.cahnrs.wsu.edu/Public/FactsheetWeb.aspx?ProblemId=776
Bacterial leaf spot: https://www.canr.msu.edu/news/bacterial_leaf_spot_on_hydrangea
There are cultural practices that can help reduce the risk of leaf diseases. Maintaining airflow around the plant is very important. Plants crowded by other plants or structures or are in need of pruning have higher relative humidity that is favorable to many disease-causing organisms. Using good sanitation by removing infected leaves and avoiding overhead watering and overfertilization can also help reduce the risk. I can't say exactly why one of your hydrangeas is affected and the others are not. Disease resistance can vary by cultivar. There are different versions of the different species of hydrangea. Also, plants very close to one another may still be experiencing slightly different conditions especially in regards to water and air circulation.
If you would like to pursue chemical intervention, it is best to positively identify the disease-causing organism. You can learn this by submitting a sample to the diagnostic lab for testing. If you have a fungal infection, you can identify the appropriate fungicide control and apply early in the season before symptoms are visible.