I did something like that once with a HPA compressor. Needless to say I was asked to stop by law enforcement.
In the way of your BB feed, I would suggest a gravity feed at a 45 degree angle back to the user instead of a spring feed. With a gravity feed, you can just keep pouring BBs in and keep firing.
In the way of the 3000psi tank, I would suggest getting a high volume pressure regulator and tune it down to 100PSI. If you were to let the whole 3000PSI out to fire BBs, not only will you run out of gas fast, it will be more than lethal.
On another note, play around with it, but make sure you have the proper protection gear. When doing your first few tests with high pressures and projectiles, make sure to wear thick gloves, eye protection and a good apron. Someone could loose a eye, or in the case of a ruptured line, a finger or eye.