In this article, a broadband WSe2-/MoS2-/SnS2-based flexible photodetector on the ITO-coated polyethylene terephthalate (PET) substrate is proposed. The economic spin coating method is utilized for the deposition of WSe2, MoS2, and SnS2. Furthermore, the Aluminum (Al) contacts are made by thermal evaporation process over the SnS2 layer. The performance of the proposed photodetector is measured in terms of the responsitivity, external quantum efficiency (EQE), and detectivity parameters at −2-V bias, and 0.118- optical excitation. The proposed photodetector shows excellent responsitivity/EQE/detectivity of 204.63 A/W/ %/ Jones at 300 nm, 248.16 A/W/ %/ Jones at 650 nm, and 123.19 A/W/ %/ Jones at 1100 nm, respectively. These results suggest that the WSe2/MoS2/SnS2 heterojunctions have a great potential for broadband detection and might be exploited in a variety of optoelectronic applications.