Over the past decades, polynitrogen chemistry has witnessed rapidly growing progress. Unfortunately and to our great surprise, eta(3)-assembled species based on the simplest polynitrogen ring (N-3) have never been reported. In this paper, we report the first successful assembly design of the long-escaped N-3-based Compounds, that is, [N3NiN3](2-), [N3M(CO)(2)N-3](q) ((M,q), = (Fe,0) (Mn,-1)), [N3M(CO)(3)](q) ((M,q) = (Co,0), (Fe,-1)), and [N3MCp](q) ((M,q) = (Ni, 0), (Co,-1)), at the density functional level. The conversion and dissociation of them need to overcome considerable barriers kinetically. To our surprise, the detailed structural, charge distribution, and orbital analyses consistently reveal a triplet polynitrogen unit, cyclic-N-3(3)-, rather than another simplest trinitrogen unit cyclic-N-1(3)+. The two unpaired spins within the naked cyclic-N-3(3)- have effectively participated in the bonding interaction with the central transition-metal atoms (here, M is Ni, Fe, Co, and Mn). Moreover, the possible experimental routes of N3CO(CO)(3) were proposed. The diradical-like polynitrogen ring, cyclic-N-3(3)-, would add to the polynitrogen family as a novel building block. The assembly-designed species await future experimental verification.