How to use Knapsack problem?
 

Basically I have a set, O consisting of N 3-tuples, <X, Y, Z>. I am trying to construct a set O' subject to the constraint that sum of all X's in O' is within some range X'Range, the average of all Y's is within some range Y'Range, and the sum of all Z's is within some range Z'Range.

This seems like a perfect situation for the knapsack problem does it not? However I am a little confused about how to do this.

Pseudocode would be great!

Steve