Fitts' law is widely used as an evaluation tool for pointing or selection tasks, evolving into diverse applications, including 3D extended reality (XR) environments like virtual, augmented, and mixed reality. Despite standards like ISO 9241:411, the application of Fitts' law varies significantly across studies, complicating comparisons and undermining the reliability of findings in 3D XR research. To address this, we conducted a systematic review of 1957 papers, focusing on 122 studies that used Fitts' law in 3D XR user experiments. Our analysis shows that over half of these studies referenced Fitts' law without thoroughly investigating throughput, movement time, or error rate. We performed an in-depth meta-analysis to examine how Fitts' law is incorporated into research. By highlighting trends and inconsistencies, and making recommendations this review aims to guide researchers in designing and performing more effective and consistent Fitts-based studies in 3D XR, enhancing the quality and impact of future research.