Mass Effect 3 is necessary to experience the Extended Cut downloadable content, players can view the new ending of the game even more quickly.
To cut out the middleman (and potentially any fresh content included in the missions prior to the end of the game), players who have completed the game can load up an auto-save that zips Shepard to the end of the game, labeled (spoilers!) "Citadel: The Return." Again, while you might miss added scenes during the final missions in the game, you'll go straight to all the explanatory stuff at the end.
It's worth noting that we performed this act of trickery on Xbox 360, so we can't confirm that this works on PS3 or PC just yet.