Long Post Incoming!
ME1 Companions
Ashley / Kaidan - Maybe - That you could stick with your romance with them if you choose nobody new gives a decent chance for them both to be companions in ME3...or at the very least have a bigger role then ME2...
Garrus - Yes - I highly doubt they will get rid of Garrus as a companion for ME3...especially since you could romance him in ME2...and that he was also in ME1...
Wrex - Doubtful - Depending on whether you killed him or not will obviously have a big role...but even if you did not I highly doubt he is going to leave his clan and what not to help you out
Tali - Yes - Yet again Bioware would be quite stupid to keep Tali out as one of your companions for ME3...
Liara - Maybe - There is a slight chance you could get her I am sure...but I would put more money on her not being a companion then being one...
ME2 Companions
Jack - Maybe - Personally I would not like to see Jack in ME3...but I am sure there are some fans out there for her...and she might have a decent shot to make it in
Grunt - Maybe - Grunt can go either way...he could continue to tag along with Shepard...or he could go back with the other krogans in their homeworld...
Jacob - Maybe - Yet again another companion I think we could do without (and I am sure most would agree here as well!) But for those who did a romance with him would feel shafted if he did not appear in ME3...
Kasumi - Doubtful - I actually thought Kasumi was quite decent for DLC companion...and it did not hurt that she was quite a good battle companion as well(At least for me on Hardcore diff) though being a DLC companion I doubt she will return...though if were lucky perhaps...
Legion - Maybe - You do not get to spend much time with Legion as you get him near the end of the game (if you are playing to save the crew at least and what not) With that being said...because of that I think there is a chance you could see him a lot more in ME3 if they choose to keep him...though he could swing either way and just go back to his people...
Miranda Lawson - Yes - I highly doubt Miranda would be left out of ME3...Shes just too iconic to get rid of. Also all the people romancing her (myself included for my paragon shep) would be quite angry if shes not in ME3 as a companion...
Mordin - Yes - I am saying Yes to Mordin...but not in the way you may think...I think he will be in ME3 but as a companion / ally...meaning I do not think you will be able to take him to battle with you...but I think instead of that random computer thing that you had to go to for upgrades...I think they may perhaps just have him take over that and stay on the Normandy 24/7.
Mornith / Samara - Maybe - I am saying maybe to her...but a very WEAK maybe...Since you can kill Samara and get Mornith as your companion I doubt they will choose her over other previous companions...however if you have Samara her being a justicar gives her a somewhat decent chance to appear as a companion...
Thane - Maybe - I like Thane a lot...but his illness may limit or take him out of ME3...hopefully he appears in someway shape or form in ME3...
Zaeed - Dobutful - Of all the people to reappear as a companion...I think Zaeed has the lowest chance to be a companion in ME3...I highly doubt will see him in any shape or form in ME3...though if we do...I am 99.9 percent sure it will not be as a companion...(Perhaps Ill eat my words one day...but I doubt it!)
Anyways thats the list to my knowledge...I imagine that ME3 will at least 3-6 new companions...and will bring back at least 2-5 companions from previous games...It remains to be seen who...(at least to me at this moment) but there you go!
Feel free to add your own insight and what not...or comment on my own and tell me how right or wrong you think I am on everyone

Edited by Tempest, 13 April 2011 - 03:27 PM.