Prune out all the dead areas and feed them…Tree-Tone for the arbs, Holly-Tone for the Rhododendron and Flower-Tone for the Magnolias.   Also check to make sure that mulch hasn’t been piled up against the trunks of the plants.  Get under there and brush the mulch away from the trunks or stems of the plants until you can see the original soil level and keep it exposed a few inches out away from the trunk.  Here’s a link describing how to feed using the Tone foods:   https://blog.timesunion.com/gardening/breakfast-in-their-beds/7457/