AB Dor, Speedy Mic, and Rst 137B are in their early post-T Tauri evolutionary phase (<100 Myr), at the age of fastest rotation in the life of late-type stars. They straddle the coronal saturation-supersaturation boundary first defined by young stars in open clusters. High-resolution Chandra X-ray spectra have been analyzed to study their coronal properties as a function of coronal activity parameters Rossby number, LX/Lbol, and a coronal temperature index. Differences between stars suggest that as supersaturation is reached the DEM slope below the temperature of peak DEM becomes shallower, while the DEM drop-off above this temperature becomes more pronounced. A larger sample comprising our three targets and 22 active stars studied in the recent literature reveals a general increase of plasma at T>~107 K toward the saturated-supersaturated boundary but a decline beyond this among supersaturated stars. The coronal Fe abundances of the stellar sample are inversely correlated with LX/Lbol, declining slowly with rising LX/Lbol, but with a much more sharp decline at LX/Lbol>~3×10-4. For dwarfs the Fe abundance is also well correlated with Rossby number. The coronal O/Fe ratios for dwarfs show a clear increase with decreasing Rossby number, apparently reaching saturation at [O/Fe]=0.5 at the coronal supersaturation boundary.