Critical applications in wireless sensor network (WSN) are real-time based applications. Therefore, users are generally interested in accessing real-time information. This is possible, if the users (called the external parties) are allowed to access the real-time data directly from the sensor nodes inside WSN and not from the base station. The sensory information from nodes are gathered periodically by the base station and so, the gathered information may not be real-time. In order to get the real-time information from the sensor nodes, the user needs to be first authorized to the sensor nodes as well as the base station so that the illegal access to nodes do not happen. In this paper, we propose a novel three-factor user authentication scheme suited for distributed WSNs. Our scheme is light-weight, because it only requires the efficient cryptographic hash function, and symmetric key encryption and decryption operations. Further, our scheme is secure against different known attacks which are proved through the rigorous informal and formal security analysis. In addition, we simulate our scheme for the formal security verification using Automated Validation of Internet Security Protocols and Applications tool. The simulation results clearly demonstrate that our scheme is secure against passive and active adversaries.