A thin film transistor, an array substrate, and a manufacturing method thereof. The manufacturing method comprises: forming a buffer layer and an active layer sequentially on a substrate, and forming an active region through a patterning process; forming a gate insulating layer and a gate electrode sequentially; forming Ni deposition openings; forming a dielectric layer having source/drain contact holes in a one-to-one correspondence with the Ni deposition openings; and forming source/drain electrodes which are connected with the active region via the source/drain contact holes and the Ni deposition openings.