Say, for instance, I want to run Tomb of Annihilation for three different groups....one monday, one wednesday, and one friday.....Can I just purchase the module once and do this or is that not possible?
Of course. Once you purchase the module you can have any number of games using it.
Just as an aside from personal experience -- take good notes and lots of them! (unless one of your players in each group does and posts them) Confusing as heck to keep different games in your head once they have diverged a bit.
Cool thing you can do is prep one game and copy it into multiple versions. Saves any work you might have to do (even thought most of the module is prepped already, it lets you tweak it once and make copies of it).
That is interesting...So for example: I can setup a map with dynamic lighting and transfer it to another game without having to configure the map all over again? I'll look into how this is done. Very cool.
Actually you would have to copy the game completely not transfer the map (unless you go pro and use the transmogrifier feature) to do what I was talking about. Build the game (or purchase the module and change it to reflect what you want in the game) then copy the game for each group that was going to run in it. This gives the Creator a fully tweaked game for each group to start with but only the work done on one game (base game or source game).