The purpose of the 3rd International Workshop on Software-Defined Sensor Networks (SDSN 2015) is to offer a forum for practitioners and researchers from diverse backgrounds to interact and exchange experiences about the design and implementation of SDSNs. The topics of interest include, but are not limited to: communication and network protocols sensor network applications Internet of things cognitive sensor networks over-the-air programming hardware design and implementation operating systems and resource management awareness computing environment and context modeling middleware systems deployment and configuration programming languages and compilers software engineering for wireless sensor networks deployment and configuration energy efficient protocols prototypes, testbeds, field experiments cloud-assisted wireless sensor network management cloud-assisted data management and analysis context sensing and context-aware computing architecture design and evaluation