There is a forum for each of those games. Rulebooks for all are available at the Avalon Hill site. Previous version discussion from several years agao.
D-Day is the weakest of the three.
Guadalcanal is the most zen of all the games. You have to think and plan but not to hard. The movement structure makes the game feel very fast. It is the only one that uses a ‘purchase units’ type of reinforcement system. Bulge and D-Day have a pre-determined system.
Bulge is the most traditional style wargame entry. It is the game that seems to be most trying to put you in the place of the actual commanders of the battle and answer the problems they faced. It is also the most radical departure in terms of mechanics.
I find the reception overall the GCNL and BOTB got quite disappointing as it has led to nothing but variations of Revised for the last six years.