What happens when you weight lift is that when you put your muscles in overload, you push your limits and deliberately tear muscle tissue. Within the next 24-48 hours, the damaged muscle tissue goes into a repair process. As it repairs, it builds a little more muscle to be able to withstand the weight you added on to your workouts. But for your muscles to effectively repair and get stronger, you must give it the fuel it needs, which is protein.
Good protein shakes you can get are muscle milk and whey protein. Do not go overboard with the protein. Aim for about one gram of protein for each pound you weigh. (For example, if you weight 180 pounds, take in 180 grams of protein). You do not have to intake all that protein just from shakes. For example, you can take in 50%-75% of the protein you need from the shakes, and the rest from foods such as meats(chicken, beef, etc), beans, protein bars, eggs, and milk.
Other than that, keep up the good work! Three days a week is an excellent plan. Anything less and your bound to have little to no progress. Anything more and your over training, which will result in no progress.