<p>Twisted trilayer transition metal dichalcogenides host two overlapping moiré patterns that can generate novel structural and electronic phenomena beyond those in bilayers. Here we show, using a continuum approach, that lattice relaxation in trilayer WSe₂ produces distinct domain arrangements and band structures in two different twist configurations. A key feature is the summation of moiré potentials onto the middle layer, resulting in potential landscapes qualitatively distinct from those in bilayers. In helical trilayers, this summation leads to Kagome flat bands, while in alternating trilayers it produces strongly confined quantum-well-like states. These characteristic band structures arise naturally from relaxation without requiring precise stacking control. In addition, the overall moiré-of-moiré pattern hosts localized boundary states at domain intersections. We further demonstrate that a moderate perpendicular electric field can change the layer character of the electronic states, enabling tunable interlayer hybridization. Our findings establish trilayer moiré systems as a versatile platform for engineering controllable electronic and excitonic behaviors.</p>