im not sure if the ultimate paragon ending can be had or not with a "canon" runthrough, but from what i saw its not worth replaying the first 2 to get. if you did that you'd be stoked for the ending and let down bigtime - the only diff before the epilogue is a brief cutscene that hints shep is (barely) alive, rather then vaporized or liquidized. shep is still alone and the crew are marooned elsewhere (even any crew with you on earth are not shown), and galactic civilisation is still likely to be destroyed - the storytelling is not good enough to be sure (which makes no sense with 2 of the endings but i guess it saved them rendering different outcomes)
it seems strange as there is so little diff between any endings, and there is no "good" ending with any closure at all. it reminded me of the DA2 ending and im not even being snarky - its totally devoid from the trilogy people just played for 50 hours or so and tacked on the end. buzz aldrin owns tho. people will be sad and annoyed.