If you do play III, and you want to get 100%, make sure you don't burn any bridges-- what I mean is you get assigned missions from people to take out somebody else you do missions for--if you take them out prior to doing all the missions for them and/or their subordinates--- you will lock yourself out of some missions making it impossible to get 100%.
I also urge you to do ALL the telephone missions on the 1st island prior to doing any missions on the 2nd island. If you don't, one of the 1st island phone missions is nearly impossible to do if you do the 1st mission on the 2nd island.
This may sound kind of confusing but I'm trying not to give you any spoilers.
One last thing. Each rampage has 2 locations, if you fail it, it moves to its 2nd location--if you don't want to search for them a 2nd time, when you find a rampage, go save and then do it. reload if you fail. The rampages in III are much harder then VC-- you have now been warned.
There is not as much side stuff in III, so it doesn't take as long to complete the game to 100%--as long as you don't burn any bridges.