The type cover will work, all the (Surface) type covers are compatible with each model, well so far anyway. Before I got enough money to get the keyboard for my Pro 3 I used my old 1st gen type cover from my Surface RT, and it worked fine. The only problem you'll have is the keyboard could look a little odd as the pre-Surface pro 3 models had an aspect ration of 16:9, to accommodate for the 3:2 ratio on the Surface 3 the tablets been decreased in width, thus the keyboard could appear longer than the tablet itself (you may need to ask other people about this as I don't have the Surface 3)
Hope this helps!