Vertically stacking two or more two-dimensional single-layer materials has been proven to be one of the most commonly used strategies in constructing van der Waals heterostructures for enhancing the electronic, optical, and photocatalytic properties of the two-dimensional materials. Based on the first-principles calculations within the framework of density functional theory (DFT), this paper systematically investigates the geometric structure, interface characteristics, optical properties, strain- and electric-field-controllable three-layer van der Waals heterojunction α-Te/InSe/γ-Te formed by single-layer insertion of the InSe semiconductor into α-Te/γ-Te heterojunction. The results indicate that the InSe semiconductor intercalation method can significantly reduce the hybridization interaction between the α-Te and γ-Te monolayers. The ground state of the three-layer α-Te/InSe/γ-Te van der Waals heterojunction exhibits a typical n-type Schottky contact and forms a potential barrier of 0.16 eV. Under a positive electric field, the three-layer α-Te/InSe/γ-Te van der Waals heterojunction may transform from an n-type Schottky contact to a p-type Schottky contact or an ohmic contact. Under a negative electric field, the three-layer α-Te/InSe/γ-Te van der Waals heterojunctioncan transit from an n-type Schottky contact to ohmic contact. The three-layer α-Te/InSe/γ-Te van der Waals heterojunction transits from an n-type Schottky contact to a p-type Schottky contact under the biaxial compressive strain, and the interface barrier height decreases with biaxial tensile strain. The maximum absorption coefficient of the three-layer α-Te/InSe/γ-Te van der Waals heterojunction is as high as 32%, which may be effectively manipulated by external biaxial strain. Under an electric field, the maximum absorption coefficient may reach 39%. The results can provide theoretical references for the design and application of novel nanoelectronic/optoelectronic devices based on three-layer α-Te/InSe/γ-Te van der Waals heterojunction.