What position(s) do you play? In any case, 3 weeks is not much time to improve significantly in any aspect of the game. I played small-time college ball, and I'm a High school coach now. I'll throw a few things out there:
If you are a position player, you will probably be asked to run the 40 or 60yard dash. Coaches love speed. Work on your start, which is the most important element in any sprint. Work on starting both from a base-running (stealing a base) stance, and track-style (hand on the ground). Some coaches will make you start from the base-running stance, but if you have a choice use track-style which is a little quicker. Work on explosion with your cross-over in BR stance and stay low for the 5 steps or so, and gradually get tall. It makes a big difference.
You will also be asked to throw obviously, no matter what position you play. If you happen to be an outfielder, work on your angles in getting to the ball quickly on ground balls to your left or right. Work on getting rid of it as quickly as possible. If the ball is in the air, work on getting back and getting behind the ball with momentum coming forward, don't drift. Also, keep your throws on line as if you were throwing through the cut-off man. Don't miss high. Long one hoppers are fine as long as they are accurate.
If you are a middle infielder, work on your angles and footwork so that your momentum is behind the ball. Move your feet and try to avoid the backhand if possible, coaches don't like it. Work on your footwork around the bag turning DP's.
If you are a 1st baseman, work on your picks.
If you are a Catcher, work on blocking balls. Have someone beat you to death for the next 3 weeks, and make sure they are spinning it to simulate breaking stuff. Coaches really appreciate a catcher who can block. No matter what your arm strength is, there's not much time to make a difference. But, you can focus on your footwork and speeding up your release. Make sure your footwork is good, and you are bringing the ball straight behind the ear with no hitches. Follow through, and don't short-arm. Be vocal and confident.
As a hitter, the game has changed with the new bat rules. It's no longer HR derby. Make sure you can lay a bunt down, and hit behind runners . If you have speed, work on your push and drag bunt for hit. Coaches will find a place for you if you can do this well, trust me. Also, don't over-swing. Hit the ball where it's pitched. Spray it around and just focus on putting a good swing on it and hitting the ball hard somewhere.
If you are a pitcher, focus on hitting the black at the knees. Keep it down and stay away from the middle of the plate. Make sure you work out of the stretch some, because most kids don't do it enough.
Good luck!