My advice is that you should do the adventure in 3 different more or less self contained adventures all with a common theme. For example if the mystery is to recover an ancient artifact that was stolen, then for the first part you could have the players track down the artifact. In the second part you could have the characters could attack the thieves hideout to reclaim the treasure, connecting this to the first and throw in a plot twist by making a ransom note to start the adventure with. In the third part you could explain to the characters that the treasure is too powerful to let it possibly fall into enemy hands so the king hires the characters to dispose of the treasure in some way, for instance throwing it into the only known pit of fiery acid in the world, which just so happens to be guarded by demons. Connect the third part to the second part by saying that the king has ordered it to be destroyed after it was found by the prince, which caused him to go mad.
Now all this is of course a simplified version of what you would be doing, but you get the jist, make one adventure with three parts, and change something small about each part to make it not seem like a continuation or spoil the previous parts.
