D-76 is normally sold as a powder to be mixed by the user. They come in 1 liter, 1 gallor, and 5 gallon mixtures. I belive they come in powder form because of the limited shelf life of the developer when mixed with water and comes in contact with air.
It would be best to ask the store where you bought the chemicals what strenght the D-76 was. It can be used straight or as a 1:1 dilution, development times will vary.
This website will be useful in finding development times needed for Tri-X and D-76;
Its also possible to resuse D-76 when used straight, you must re-adjust developing times after each use. On that note, its easier to mix a 1:1 batch and discard after use.
The 3 parts to 7 parts dilution on the fixer usually refers to the intended use. For film development: 1 parts(fixer) + 3 parts(water) is common. For paper printing: 1 parts(fixer) + 7 parts(water). Fixer can be reused multiple times, as mentioned earlier an easy way to test is to drop a small piece of film into the fixer noting how long it takes to clear, then double that time just to be safe.
As to storage, Its perfectly fine the way they are, as seen in your picture. Its best to minimize the chemicals contact with light, air, and too much heat. This is to help prolong its active life. So a dark place would be better than next to a window.
When mixed D-76 has a shelf life ranging from weeks to months depending on how its stored, the less air it comes in contact with the better. The pdf from Kodak should have all the information. For fixer, Ive had a bottle where I mix working strenght fixer (1+3) from for about 3 months now and its worked fine, not much more I can tell you from that.
These websites are useful and contain a lot of information.