You could add aluminum sulfate to address the iron chlorosis; carefully remove it out of the pot, check for circling roots, cut the roots every 1 or 2" if there are any circling the pot, put it back adding new potting mix for acid loving plants. All the constant watering needed by potted shrubs seems to have dropped the minerals and soil level down a bit.

After 3 years, it does need repotting. But wait until the plant is dormant before you do that. Do you fertilize? Anything grown in a container will need routine fertilization....and far more frequently than the same plant grown in the ground. I dose all my containerized hydrangeas with Osmocote Plus in the spring and follow up with Miracle Gro for Acid Lovers at half strength once a month during the growing season.

What gardengal said... This happened to me when I neglected to repot a hydrangea one spring. It developed chlorosis that I tried to fight with liquid iron, fertilizer, you name it. Nothing helped. I finally repotted it in the middle of summer (delicately) and it improved. Your potting mix has broken down with time and is completely depleted at this point. Fresh potting soil in a larger container, plus regular fertilization will do the trick.