Trust establishment in pervasive environments, where distributed and resource-limited devices need to establish secure connections, has triggered much effort in the last years. One way to bring trust into a Personal Network (PN) is the use of imprinting, which consists in an agreement on a shared secret without any prior knowledge, giving expression to the trust relationship that links a pair of devices. In this paper, we introduce the PN Formation Protocol (PFP), an imprinting protocol that is extensible to three parties. We considered the use of imprinting through an out-of-band channel performed by the user itself in the bipartite version and an autonomic and collaborative one in the tripartite version of the protocol. We describe its implementation, test results as well as the proof of its security goals through an automatic protocol analyzer.