In wireless sensor networks (WSNs), providing source node with sink position is an essential principle for geographic routing protocols. Previous works have only focused on the problem of sink localization in a 2-D sensing field while that of 3-D WSNs has received little attention. Providing sink location service with low control overhead and energy consumption is a challenging issue in 3-D WSNs area. In this paper, we propose a unital design based sink location service (UDSL) for WSNs. In this scheme, sink location announcement (SLA) and sink location query (SLQ) packets are forwarded along two paths or blocks. The node located at the intersection of the two paths sends the sink position to the source. In the proposed method, SLA and SLQ messages are constructed using unital design blocks. For this purpose, a mapping from unital design to sink location service has been proposed. However, this basic mapping does not guarantee an intersection of paths,therefore, we propose an enhanced UDSL providing 100% probability of intersection. In order to analyze the proposed scheme’s performance, extensive WSNs simulations and experiments have been conducted. The results indicate that UDSL provides reasonable performance in terms of hop counts, path length, and energy consumption for providing sink location service.