Simultaneous data from two interferometers separated by 16 km and synchronized within 100 ns was collected for a thunderstorm near Langmuir Lab on October 23, 2018. Analysis via triangulation followed by a least-squares fit to time of arrival across all six antennae produced a three-dimensional interferometer data set (3DINTF). Simultaneous Lightning Mapping Array (LMA) data enabled an independent calculation of 3DINTF accuracy, yielding a median location uncertainty of 200 m. This is the most accurate verified result to date for a two-station interferometer. The 3D data allowed profiling the velocity of multiple dart leaders and K leaders that followed the same channel. 3D velocities calculated from the in-cloud initiation site to ground ranged from 3x10^6 m/s to 20x10^6 m/s. Average velocity generally increased with subsequent leaders, consistent with increased conditioning of the channel. Also, all leaders showed a factor of two to three decrease in velocity as they proceeded over 15 km of channel. We speculate that the velocity decrease is consistent with energy lost in the reionization of the channel at the leader tip. This paper includes an appendix providing details of the triangulation technique used.