Three weeks in Shenyang?! yes, it is too long and enough for you to visit many places around it and even some place a bit further. To see the border area with NOrth Koera, you can get to Dandong, by train from Shenyang. Centered in Shenyang, you can go north to Changchun, Jilin, Harbin, south and south west to Dalian, Beijing and Tianjin. Well arrange your days and have fun.