The IEEE VR 2017 Workshop on Perceptual and Cognitive Issues in AR (PERCAR) is the third installment of the previous successful PERCAR workshops held at VR 2015 and VR 2016. The crux of this workshop is the creation of a better understanding of the various perceptual and cognitive issues that inform and constrain the design of effective augmented reality systems. There is neither an in-depth overview of these factors, nor well-founded knowledge on most effects as gained through formal validation. In particular long-term usage effects are inadequately understood. However, mobile platforms and emerging display hardware (“glasses”) ignite the number of users, as well as the system usage duration. To fulfill usability needs, a thorough understanding of perceptual and intertwined cognitive factors is highly needed by both research and industry: issues such as depth misinterpretation, object relationship mismatches and information overload can severely limit usability of applications, or even pose risks on its usage. Based on the gained knowledge, for example new interactive visualization and view management techniques can be iteratively defined, developed and validated, optimized to be congruent with human capabilities and limitations en route to more usable AR application interfaces.