It is well-known that many networks follow a power-law degree distribution; however, the factors that influence the formation of their distributions are still unclear. How can one model the connection between individual actions and network distributions? How can one explain the formation of group phenomena and their evolutionary patterns? In this paper, we propose a unified framework, M3D, to model human dynamics in social networks from three perspectives: macro, meso, and micro. At the micro-level, we seek to capture the way in which an individual user decides whether to perform an action. At the meso-level, we study how group behavior develops and evolves over time, based on individual actions. At the macro-level, we try to understand how network distributions such as power-law (or heavy-tailed phenomena) can be explained by group behavior. We provide theoretical analysis for the proposed framework, and discuss the connection of our framework with existing work. The framework offers a new, flexible way to explain the interplay between individual user actions and network distributions, and can benefit many applications. To model heavy-tailed distributions from partially observed individual actions and to predict the formation of group behaviors, we apply M3D to three different genres of networks: Tencent Weibo, Citation, and Flickr. We also use information-burst prediction as a particular application to quantitatively evaluate the predictive power of the proposed framework. Our results on the Weibo indicate that M3D's prediction performance exceeds that of several alternative methods by up to 30\%.