Re: 3.0 I/O 100 Hour service requirements
Click here and you can download the OEM owners manual for your engine. It has the maintenance list in it.
As Chris said, that list is a starting point, and basically a MINIMUM needed for your engine and drive.
Impellers on the Gen II drives in fresh water can probably survive 5 years, but I don't think I would let mine go over 4. Bellows are usually good for 5 years, the price really goes up if they fail and you have to replace drive parts.
Things like pulling the outdrive and checking alignment is a yearly thing also. Again, cheaper than the alternative is something on the engine comes loose and you don't notice it till you have to pull the engine for coupler replacement.
Have a look around
HERE you will probably find lots of information that may answer some questions you don't have yet.