There is disclosed a method of manufacturing an array substrate, the method including a step of forming thin film transistors on a substrate; wherein the step of forming the thin film transistors on the substrate includes: forming a first electrically conductive layer on the substrate; forming an insulating layer on the first electrically conductive layer; forming at least one common holes in the insulating layer to communicate with the first electrically conductive layer; forming a first connection portion, which is made of the same material as a second electrically conductive layer, in the at least one of the at least one common holes while forming the second electrically conductive layer on the insulating layer by using a single process, the first connection portion being in electrical contact with the first electrically conductive layer. In addition, there is disclosed an array substrate manufactured by the above method and a display device including the array substrate.