Galaxy clusters provide a unique probe of the late-time cosmic structure and serve as a powerful independent test of the ΛCDM model. This work presents the first set of cosmological constraints derived with ∼16, 000 optically selected redMaPPer clusters across nearly 5000 deg2 using DES year 3 datasets. Our analysis leverages a consistent modeling framework for galaxy cluster cosmology and DES-Y3 joint analyses of galaxy clustering and weak lensing (3×2pt), ensuring direct comparability with the DES-Y3 3×2pt analysis. We obtain constraints of S8=0.864±0.035 and Ωm=0.265-0.031+0.019 from the cluster-based data vector. We find that cluster constraints and 3×2pt constraints are consistent under the ΛCDM model with a posterior predictive distribution (PPD) value of 0.53. The consistency between clusters and 3×2pt provides a stringent test of ΛCDM across different mass and spatial scales. Jointly analyzing clusters with 3×2pt further improves cosmological constraints, yielding S8=0.811-0.020+0.022 and Ωm=0.294-0.033+0.022, a 24% improvement in the Ωm-S8 figure of merit over 3×2pt alone. Moreover, we find no significant deviation from the Planck CMB constraints with a probability to exceed (PTE) value of 0.6, significantly reducing previous S8 tension claims. Finally, combining DES 3×2pt, DES clusters, and Planck CMB places an upper limit on the sum of neutrino masses of ∑mν<0.26 eV at 95% confidence under the ΛCDM model. These results establish optically selected clusters as a key cosmological probe and pave the way for cluster-based analyses in upcoming stage-IV surveys such as LSST, Euclid, and Roman.