You can't fully grasp the entire scope by just playing through the third installment. The third game is a medium-sized part of the M&M universe, and completing it won't give you a complete understanding. To achieve that, you need to play through all the parts of the RPG series, carefully reading the NPC dialogues and delving into the plot, all four parts of the strategy series with their expansions, read the official books about M&M, and play through the three parts of the action series. And if you're too lazy to do all that, you can read a single link that I provided, and additionally search the internet, play parts of some series, and watch videos. That's what I did, and I "fully grasped the entire scope."