It may be too advanced to help but I would spray now with Neem or Ultrafine horticultural oil. When the plant dies back in the fall I would remove all plant material to the garbage and spray the ground well with Bordeaux mix. In the spring when the growth first starts I would do the Bordeaux again. Al

Have they had more blooms than that in the 6 years they have been planted? Besides what calistoga suggested above, might they need to be lifted and planted a little more shallow?

Sue

They have had dozens of flowers in the past several years, so the appearance of only three is a major change.

If I wait til fall to cut them back, will they infect nearby plants? Would it be better to cut them back now?

Your peony will survive if cut back now, and to prevent the spread of powdery mildew to other susceptible plants nearby that might be a good idea. Care should be taken to bag the mildewed plant material carefully to contain the spores. Now I would spray the cut back plant and the ground generously with a fungicide. Al