Should i save Miranda, Jacob and Grunt?

I would complete Grunt's and Miranda's loyalty missions. The reason is because without completing both, they will die in ME3. Jacob's loyalty mission does not have to be completed. If he survives ME2, he will survive ME3 if you complete the mission involving rescuing the ex-Cerberus scientists.

If you let Grunt die in ME2, his replacement will be another krogan, Urdnot Dagg. Good character. If Miranda die's in ME2, she is not replaced in ME3. Just get a few more lines of dialogue with her sister. Jacob is replaced by another guy doing the samething except he won't have the dialogue that Jacob would have.

I always save all three of them, despite finding Jacob about as interesting as wet paper maché at the best of times. They don't have huge story impact, but they do have great moments during the Citadel DLC.

Miranda: She's my favourite apart from Garrus. If you don't do her LM she will die in ME3 regardless of how careful you are. Actually, I think Miranda is the trickiest to keep alive. She has a bit of a ridiculous laundry list. *rolls eyes* To avoid too many spoilers: read all dossiers and emails you get. Agree to meet with her each time she asks, and agree to her requests. Also give her a warning when the option arises during one of the conversations. During the final confrontation on Horizon, choose a Paragon or Renegade option. I can give specifics if you like.

Grunt: Grunt's cameo is awesome, and he's also hilarious during the Citadel DLC. If you make a particular choice during that cameo mission, Grunt will die IF his loyalty is not secured. If you choose the other option he can live regardless.

Jacob: I think once he's loyal, he will survive provided you do the mission to save the ex-Cerberus scientists. I don't care for him, so I don't actually know.

and if she does survive but is not loyal, you will not be able to save her no matter what you do/decide in ME3.