Answer: It's always interesting to watch how plants develop and try to account for the differences in a grouping like this.
Any time there is dead wood it should be removed promptly, so pruning the middle plant was fine. The question though is why that occurred. You might inspect it carefully and notice that there was some damage to the plant near the base that caused a portion to die back like that, or that there was damage to the roots corresponding with the dieback. If the plant is grafted, perhaps the graft is damaged.
Otherwise, I would suspect a rooting problem. This could be caused by unequal watering, or vole damage, or by poor rooting due to encircling roots for example, where the roots continue to grow in an circle rather than spreading out into the surrounding soil. You might dig down carefully and see if you can find a reason.
As far as the black spots, this is probably "black spot" -- a fungal infection. Removing the affected leaves as soon as you notice it is a good idea. Black spot can not be cured once it shows, although it can be prevented. This winter, strip any remaining leaves off of all three of the plants and replace the top layer of mulch to try to prevent a reoccurrence next summer. Here is some additional information about black spot and general care for these roses you may find helpful.
Unfortunately, many rose varieties have little or no resistance to black spot. Innate susceptibility plus the weather as well as cultural conditions can all contribute to it appearing and to its severity in a given year.
This is a fungal infection. Damp conditions would of course encourage it to develop. Once black spot appears for the season, it is difficult to control. In general, remove affected leaves as soon as they appear and then replace the mulch immediately beneath the plant to try to keep the disease spores from splashing back up onto it. Keep the plant lightly thinned to promote air circulation and make sure it is planted in a location with good air circulation and in enough sun. If you water, avoid watering the foliage and water in the morning rather than at night. Finally, each winter, remove all of the leaves from the plant and dispose of them. Then replace the mulch beneath the plants. Also remove any prunings from the area. These steps should help limit reinfection.
Once you see signs of it developing, you can pick off the affected leaves, and consider spraying. People have reported success using products containing neem oil or containing sulfur (read and follow the label instructions carefully), or using a home-made spray containing one part milk and two to three parts water. Although some people also report success using a baking soda based spray, it can burn the foliage on some sensitive roses and according to the American Rose Society is better as a preventive than a treatment. It is usually a good idea to test spray a few leaves, wait a few days, and make sure there is no adverse reaction prior to spraying the entire plant -- some roses are extremely sensitive to sprays of any kind.
If your plants are in pots, you may also want to spray the pots and surrounding patio or deck to try to keep the spores from splashing back up onto the plants. (Test and make sure the spray will not damage these surfaces.)
Unfortunately, if you have a plant that is particularly susceptible to black spot in your humid summer climate, you may never get rid of it permanently without a regular spray routine plus some cooperation from the weather and maybe some extra good luck.
General Care Tips
Roses need full sun for at least half of the day (including the hour of noon), a location with good air circulation, and an evenly moist but well drained soil, meaning not saturated or sopping wet and never bone dry.
You may need to water them during dry spells. To know if you need to water, dig down and check the soil with your finger. If it is still damp, do not water yet. When you water, water slowly and deeply so it soaks down in and encourages deep rooting. This is better than watering lightly every day. After watering, wait a few hours and then dig down to see how far the water went; sometimes this can be surprising.
Using an organic mulch year round helps keep the soil more evenly moist, holds down weeds, and also helps feed the soil slowly over time as it breaks down. Apply the mulch in a flat layer over the root area, several inches thick, but do not allow it to touch the stems.
You should also feed your roses. In spring and again in early summer apply a top dressing of good quality compost and/or an application of general purpose granular or slow release fertilizer with an analysis of 10-10-10 or similar proportions. Read and follow the label instructions.
This is a low maintenance rose, but you should prune it back each spring by about half its size to remove winter damage and bring it to a somewhat symmetrical appearance. You may do this by cutting individual canes or by simply shearing it off.
I hope this helps.
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