Here is an excellent article written by Matthew Kory back in January regarding the 3 year window. He explains it much better than I ever could.
http://boston.locals.baseballprospectus.com/2017/01/20/dave-dombrowski-and-the-three-year-window/
In particular, the following paragraph rings so true with me:
"Ideally a baseball organization rolls along like a bus. It stops every now and again for some people to get off, but usually others get on as well. The job of the GM is to keep all the seats filled with quality players. The perfect case scenario would see an almost seamless addition of players from the minor league system to the major league roster, with the occasional free agent signing as needed. The Red Sox were pretty close to that ideal, about as close as could reasonably be asked, when Dombrowski took over."