The evolution in the Internet of Things domain has created a complex ecosystem populated by various so-called “the Internet of Things (IoT) platforms” that provide access to a broad range of connected devices and/or sensors “things”. These IoT platforms typically promote their specific interfaces and information models, and often adopt non-standard, sometimes fully proprietary, protocols to control a variety of things. This fragmentation and missing interoperability currently prevents the emergence of cooperative IoT ecosystems in which applications can be built so as to use things operated by various IoT platforms, or to enable things operated by different platforms to interact. The 3nd Workshop on Interoperability and Open-Source Solutions for the Internet of Things focuses on this challenge. The workshop is co-organized by the symbIoTe and BIG IoT consortiums as part of the European Platform Initiative (IoT-EPI).