Building plume is important for ventilation and pollutants dispersion along and above buildings in an urban canopy layer. This study fundamentally explores the merging process and temporal penetration of triple uniformly distributed starting building plumes, with a focus of the spacing effect on near-field flow dynamics. Instantaneous velocity and vorticity distributions, penetrating velocities, and stream-wise penetrated heights are quantitatively examined using 2-D particle image velocimetry (PIV) measurements at spacing ratios S/W (building spacing/building width) of 0.2, 0.5, and 1.0. We identified a four-stage merging progress and captured three main spacing-induced merging features. A compact layout at S/W = 0.2 introduces a strong upward channel flow. The wall flows beside the channel tend to draw together first and the unstable channel flow determines the flow pattern transition. In contrast, wider layouts at S/W = 0.5 and 1.0 exhibit intensive downward flow. The wall flows tend to exhibit self-merging initially and the downstream natural swaying motion dominates the merged pattern variations. Merging effect and buoyancy force jointly determine the temporal penetrating velocities. Temporal series of maximum axial velocities above the middle source fits into a power law profile at S/W = 0.2 but a linear function of time at S/W = 0.5 and 1.0. The normalized penetrated heights at S/W = 1.0 are notably faster than in the other two cases before the normalized time is at 3.00 probably because the weaker entrainment and interaction with neighbors lead to less energy and momentum dissipation, quicker self-merging, and faster penetration.