Assembling two-dimensional (2D) MXene nanosheets into stable three-dimensional (3D) architectures is crucial for unlocking their potential in energy storage and adsorption applications. However, the inability of directional forces to overcome strong interlayer interactions under ambient conditions fundamentally limits the formation of stable 3D MXene structures. Herein, we unveil a universal confined-ion strategy that drives the 2D-to-3D transition inside natural wood microreactors. Potassium ions neutralize the surface charge of MXene nanosheets, triggering the formation of petal-like units, and spatial confinement guides their non-planar assembly into rose-like assemblies with tunable dimensions from 1.0 to 10.2 μm. Compared with 2D MXene, the resulting 3D MXene assemblies exhibit a 16.1-fold larger specific surface area and outstanding electrochemical performance, delivering gravimetric and areal capacitances of 895.9 F·g⁻¹ and 4.3 F·cm⁻² with a retention of 100% after 10,000 cycles, respectively. The MXene assemblies also exhibit excellent multifunctionality, achieving a solar evaporation rate of 1.9 kg·m⁻²·h⁻¹ and a dye removal efficiency of 99.1%. Importantly, this strategy is universal across a range of MXenes, offering a scalable and versatile platform for constructing robust 3D architectures from MXene.