Decision wise, there's always some side-quests and stuff that help if you import.. war assets and stuff.. plus some people might not be alive.. things like that
Story wise, as long as you know that "big scary aliens shaped like cuttlefish are coming to destroy the galaxy", you should be fine.. your playthrough would be limited though, i'd recommend picking up the first two but they aren't absolutely necessary..
ME3 is definitely a fun game on its own, but you're missing out on a lot of the storyline by playing it aloneónot just in the events of the previous games but interactions based on choices in ME1 and ME2, as well as several sidequests requiring an imported game in order to get the best resolution.
It should still be a good experience, but much better after importing from the previous games.
This is the one game where major and minor choices in the first can effect stuff in the second and third game. Yes, you should play them all in order. Not to mention the first is my favorite, then the second, and have yet to beat the third. And when I say the first is my fav, I mean by far.
For thousands of years, people have speculated an event of the greatest magnitude will take place on December 21st, 2012....and its only a few months away