1. A program method of a multi-bit memory device, the program method comprising: programming first page data in a first region of a memory cell array; storing the first page data in a first buffer of a page buffer; programming second page data in the first region of the memory cell array; storing the second page data in a third buffer of the page buffer; programming third page data in the first region of the memory cell array; transferring the second page data stored in the third buffer to a second buffer of the page buffer and then storing the third page data in the third buffer; performing, based on the first to third page data stored in the page buffer, a first programming operation on a second region of the memory cell array.