people always say throw out opened containers, but i personally feel that is wasteful, and unwarranted.
brake fluid is hygroscopic, which means it pulls moisture out of the air if exposed. as long as you dont leave the cap off for any extended period of time, or leave the cap loose, it will not absorb water. after opening and using a portion, screw the cap on tight, and squeeze the bottle in your hand. if air comes out, its not sealed air tight, if you squeeze and it holds the air in, no moisture will get in. it will have a long shelf life as long as its sealed up well. but when i doubt, i wouldnt risk it, if you cant remember what you used it for when you opened it toss it out. but there is no need to be over paranoid about it being opened previously as long as its stored well.
the cap of your master cylinder says "use only DOT 3 brake fluid from a sealed container"... i consider a previously opened and resealed container "sealed" as in not left open on a shelf. your brake system is much less sealed than a tightly capped bottle, and most manufacturers give what, 2 year intervals on brake fluid change?