Do you have any information or click on videos concerning a easy motion offense for 3-4th graders starting at the 3 point line?

I would keep this very simple.... pass and cut.... take a look at these.... if it were me, I would use the 5 out motion.

http://www.breakthroughbasketball.com/offense/motion-offenses.html

This is more of a continuity offense, but if you give players a little freedom to deviate, it's a good, simple, and flexible offense:
http://www.breakthroughbasketball.com/pr/perkins-open-post-offense.html

Once you add the screen away option, I would allow players to stop on the post and look for them inside. This gives all your young players the opportunity to catch the ball inside and on the perimeter.

Here are some simple rules you can use in a free form youth motion offense:
http://www.breakthroughbasketball.com/offense/motion-offense-youth-basketball.html