When you are knocking down a 3, you want to make sure that your eyes are locked on the basket and that you are not trying to be distracted on the point of release. On the point of release just make sure that you can picture the ball before hand going in.
If you can picture the ball going in before hand then you have a much better chance of making the basket than you do of missing. When you are shooting, it is also important that you are very calm and relaxed while shooting so then you do not miss the shot.
Of course taking the shot is the best thing, but what if you feel you will get blocked. If you feel you will get blocked there are two options instead. You can either, pump fake to get him jumping and then shoot or you can pump fake, either drive or floater depending on the situation.
You also want to make sure that you practice your threes on a daily. To knock down threes you surely do have to practice them, and when you practice them, you will see that more will start falling.
Make sure that when you are taking a three you are confident that it is going in. If you are not confident try shooting at the mid range point and then work your way up. There should also be videos on how Stephen Curry the greatest 3 point shooter shot the ball.