The ICC has put out a series of animated videos to help you understand some of the games rules, narrated by Stephen Fry:
The video does not explain the no ball for three behind square. There may be no more than two fielders, other than the wicket-keeper, standing in the quadrant of the field behind square leg. See Bodyline for details on one reason this rule exists.
The expectation is JCUH players will not attempt this form of dismissal. If the non-striker is backing up too far, a warning will be given.
These explanations should be viewed in conjunction with your junior grade's playing rules which take precedent.
Junior Cricket Upper Hutt, established 1996 : firstname.lastname@example.org://www.facebook.com/JuniorCricketUpperHutt/