The game is definitely better played with all three. It gives you the chance to see how "the other half" lives. For example, if you didn't main a Titan, the first time you shoulder charge someone in PvP, or discover the sheer joy of Actium War Rig paired with Sweet Business, Heir Apparent, or Xenophage in PvE, is awesome. The same goes with the other classes. They are different enough that using each is a unique experience. As for power levelling, your first character is the hardest to level. Your second and third inherit your high level weapons from your first character, so they level up much faster, making activities easier the second and third time around.