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The diversity of Type II supernovae (SNe II) is thought to be driven mainlyby differences in their progenitor's hydrogen-rich (H-rich) envelope mass, withSNe IIP having long plateaus ($\sim100$ days) and the most massive H-richenvelopes. However, it is an ongoing mystery why SNe II with short plateaus(tens of days) are rarely seen. Here, we present optical/near-infraredphotometric and spectroscopic observations of luminous Type II short-plateauSNe 2006Y, 2006ai, and 2016egz. Their plateaus of about $50$-$70$ days andluminous optical peaks ($\lesssim-18.4$ mag) indicate significant pre-explosionmass loss resulting in partially stripped H-rich envelopes and earlycircumstellar material (CSM) interaction. We compute a large grid ofMESA+STELLA single-star progenitor and light-curve models with variousprogenitor zero-age main-sequence (ZAMS) masses, mass-loss efficiencies,explosion energies, $^{56}$Ni masses, and CSM densities. Our model grid shows acontinuous population of SNe IIP-IIL-IIb-like light-curve morphology indescending order of H-rich envelope mass. With large $^{56}$Ni masses($\gtrsim0.05\,M_\odot$), short-plateau SNe II lie in a confined parameterspace as a transitional class between SNe IIL and IIb. For SNe 2006Y, 2006ai,and 2016egz, our findings suggest high-mass red supergiant (RSG) progenitors($M_{\rm ZAMS} \simeq 18$-$22\,M_{\odot}$) with small H-rich envelope masses($M_{\rm H_{\rm env}} \simeq 1.7\,M_{\odot}$) that have experienced enhancedmass loss ($\dot{M} \simeq 10^{-2}\,M_{\odot}\,{\rm yr}^{-1}$) for the last fewdecades before the explosion. If high-mass RSGs result in rare short-plateauSNe II, then these events might ease some of the apparent underrepresentationof higher-luminosity RSGs in observed SN II progenitor samples.

Luminous Type II Short-Plateau Supernovae 2006Y, 2006ai, and 2016egz: A Transitional Class from Stripped Massive Red Supergiants