So lots of characters in Mass Effect can live or die and their status is imported if I have Mass Effect 2 save data. Are all of these characters "alive" if I start a fresh game in Mass Effect 3 without importing any data? Or are they not all present in the game as squad mates?
First off, the only returning squadmates are Liara (who can not have died), Ashley/ Kaidan (one of whom must have died), and Garrus and Tali, who the game assumes to have lived.
As for other notable life and death descisions for non squad members, the game assumes:
Wrex was killed on Virmire.
Grunt was never awoken from his tank.
Samara died at Morinths hands, though Morinth does not appear directly either.
Legion was never activated.
Jack and Thane died during the suicide mission.
Miranda and Jacob survived the suicide mission.
Of the Normandy's crew, only Joker and Doctor Chakwas survived the suicide mission.
According to this GameFront article, there are significant roster deletions by not importing a ME2 save - you're certainly not getting credit for the "best" ending.
For instance, if you don't import a save:
- Jack and Thane will die on the suicide mission
- Mordin, Jacob, Garrus, Tali and Miranda are part of the crew at the end of the game and survive the suicide mission.
- Grunt, Legion, and Samara are considered to have not been recruited by Shepard.