In this paper we investigate the formation of interstitial nitrogen trimers N3 which have been suggested as a fast-diffusing species in silicon recently. Out-diffusion profiles of nitro-gen show the involvement of at least two independent nitrogen related defects in the diffusion process depending on the nitrogen concentration at different depths of the sample. When the nitrogen concentration is small it is proposed that nitrogen trimers are formed in a two step process. We present the structural properties of such a defect using density functional theory and examine the energetics of the two proposed reactions leading to the formation of N3.