Hmm... well, IMHO, I don't need every former companion to return as a permanent one in ME3. And I plan to use my saves where all survived ME2 as my canon run for ME3.
So as much as I like Wrex, Garrus, Mordin and Liara, in particular, I hope that they are not permanent companions.
I think that with what they can do, they should be taking care of business on their own, not in Shepard's Shadow. But I can see Garrus and Mordin staying on the Normandy, Garrus commanding his own two-man squad and Mordin remaining as the Chief Science Officer.
Then Kasumi is a toss-up with that data she has, but her skills seem more suited working solo, like Thane anyway, so I can seem them working with Liara, to help Shepard in her role established in ME2.
If Miranda stays with Shep, I think her role is best served as XO on the Normandy, keeping contact with any away teams and feeding Shep Intel from Cerberus contacts, Hackett, Anderson and Liara.
Also I figure that Tali and Legion would bridge the gap between the Geth and Quarians, so I hope they don't remain permanently glued to Shep's side either.
Then Ash/Kaidan I can see as a temp squad companion, but I hope that the time spent away from Shepard allows them to work outside of Shep's shadow in ME3.
So there are only a handful of former companions that I would think are appropriate to return as purely subordinate companions again.
Jacob is a professional, and I figure he can remain a permanent companion, but he too can be at least a squad leader like Garrus. I can imagine Shep commanding Alpha Team, Garrus commanding Bravo Team and Jacob commanding Charlie Team on missions. Since we are supposed to fighting a war in ME3 I can hope that the scale of combat increases. Who wouldn't want to hear Garrus say scoped and dropped from a Gunship!
Grunt seems like he can return as a permanent companion, especially since he views Shepard as his Battlemaster.
I figure that Zaeed will have an interest in fighting along side Shepard, if Vido is dead, so I can see him as a conditional returning companion.
Samara is another I figure as a conditional returning companion with Morinth dead and Shep being Paragon.
And maybe Jack could easily stick with Shepard. But possibly depending on her relationship (romance being only one facet of that relationship) with Shep, and how her loyalty mission was used to help her get a grip on her memories of her youth.
So in general, I don't think that Romantic Relationships will keep all of those companions that could be romanced, glued to Shepard. At least I hope not and that seems a bit silly to me IMHO if it did happen.
Then potentially adding that Vega dude there seems to be plenty of options for companions for Shepard in ME3.
I just hope that Shepard's former companions simply continue growing as individuals, and improving the story for ME3!