Finished all three starting campaigns in the original package, yet none of the new campaigns will unlock. I read a previous topic where someone said I should load the savegames I've done after completion; I have saved after each and every campaign but there is never a new campaign available even when 1 of the 3 are checked (even if I load all 3 completed campaign saves after eachother, only 1 campaign is ever checked.)

According to the Heroes 3 FAQ at http://faqs.lokigames.com/heroes3faq.html
10. Gameplay
Q : Do I need to play the campaigns in one series, or can I start a new game for each of the first three campaigns?
A : If you want to access the later campaigns, you will need a save game which includes successfully playing through all three of the first campaigns at three scenarios each. At the end of each specific campaign, you will be offered the chance to save your game. Do so, and continue playing from that game (or re-load the game later to continue), and then choose a new first-tier campaign to play.
If you do not play all three first-tier campaigns (Long Live the Queen, Dungeons and Devils, and Spoils of War) in the same game, you will not unlock the later campaigns! Unfortunately, it is not possible to combine the saved campaigns if they have been played separately.