Galaxy clusters provide a unique probe of the late-time cosmic structure and serve as a powerful independent test of the Λ⁢CDM 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 ×2⁢pt), ensuring direct comparability with the DES-Y3 3 ×2⁢pt analysis. We obtain constraints  of 𝑆8 =0.864 ±0.035 and Ωm =0.26⁢5+0.019−0.031 from the cluster-based data vector. We find that cluster constraints and 3 ×2⁢pt constraints are consistent under the Λ⁢CDM model with a posterior predictive distribution (PPD) value of 0.53. The consistency between clusters and 3 ×2⁢pt provides a stringent test of Λ⁢CDM across different mass and spatial scales. Jointly analyzing clusters with 3 ×2⁢pt further improves cosmological constraints, yielding 𝑆8 =0.81⁢1+0.022−0.020 and Ωm =0.29⁢4+0.022−0.033, a 24% improvement in the Ωm −𝑆8 figure of merit over 3 ×2⁢pt 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 ×2⁢pt, 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 Λ⁢CDM 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.