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The superconducting magnet group at Lawrence Berkeley National Laboratory has been developing racetrack coil technology for economical, high-field accelerator magnets from brittle superconductor. Recent tests have demonstrated 1) robust, reusable, double-layer, flat racetrack, wind and react Nb3Sn coils, 2) a reusable, easily assembled coil-support structure that can minimize conductor movement and 3) nearly 15 T dipole fields, without conductor degradation. RD3c was our first attempt to compare measured and calculated field harmonics. A single-layer, Nb3Sn, flat racetrack inner-coil was wound on both sides of a bore-plate and reacted and encapsulated (as before in RD3b). Hard coil-spacers were wound into the inner coils to adjust the geometric field harmonics and identify any problems from hard-spacers. The resulting insert coil-module was sandwiched between existing outer-coil modules, and pre-stressed within the previous yoke and shell structure. The bore-plate was thick enough to protect the rotating coil and its anti-cryostat from this pre-stress. Magnet training started at 77% of the un-degraded short-sample current, and progressed in 15 quenches to 92%, high enough to make realistic, saturated iron magnetic measurements. Although there was considerable training, the multipole dependencies correlated well with calculations. A sub-set of the tests are reported and discussed.