Mass Effect 3 Squadmates

Mass Effect 3 was released in March 2012, and Commander Shephard's third mission against the reapers brings some new faces to his squad, and some old ones. Bioware has said they want to keep the story more focused, so there are less squadmates in Mass Effect 3 than in Mass Effect 2. The following are squadmates who will stick with you throughout your career (after you've recruited them, in some cases)

Ashley Williams
EDI
Garrus Vakarian
James Vega
Kaidan Alenko
Liara T'Soni
Tali'Zorah vas Normandy
The Prothean (via DLC)

Additionally, the following members have been confirmed to be included (assuming they survived the first two games) in some capacity or other, though they work in an assistant role rather than as squad members.

Conrad Verner
Jack
Jacob Taylor
Joker
Legion
Miranda Lawson
Mordin Solus
Thane Krios
Urdnot Wrex
Zaeed Massani

Aside from Kaidan, who was in your squad from the beginning, Ashley was the first member recruited by Shephard in Mass Effect.  Assuming she survived the Virmire mission in Mass Effect Ashley made a brief appearance in Mass Effect 2, but did not join Shephard's squad.  In between Mass Effect 2 and Mass Effect 3 Ashley will become a council spectre and will go from being an enlisted member of the Alliance military, to an officer with the rank of Second Lieutenant.

In Mass Effect Ashley was a soldier with a wide range of weapon skills.  That will probably continue to be her focus, but spectre training may grant her a few other skills.

Ashley was a romance option for male Shephard in the first game, and the player could choose to forgo other romantic options in Mass Effect 2 in favor of keeping Ashley as their romantic interest despite her not appearing much in the game.  A romance with Ashley will be an option in Mass Effect 3.

While previously confined to the SR-2 Normandy in Mass Effect 2, the sentient AI "EDI" gains control of a template body in Mass Effect 3 that she can use to join Shepard and crew on missions. She has a unique sense of humor and has earned the mistrust of ship crewmen that haven't "met" her yet, but her loyalty to the team is proven time and again. Although AI are frowned upon in the Mass Effect universe, EDI seems to buck that trend and is amicable with all species (though she will sometimes pretend to want to destroy all life... probably).

Garrus returns for the third time to join Shephard to save the galaxy.  In Mass Effect Garrus was a disgruntled security officer who joins Shephard to cut through all the red tape.  In Mass Effect 2 Garrus sets out to the lawless space station Omega to deal his own brand of justice under the guise of Archangel, before Shephard recruits him for the next mission.  Based on how the player did at the end of Mass Effect 2, it's possible that Garrus did not survive the final assault, so his appearance in Mass Effect 3 is dependent upon that.

Garrus is a turian soldier with some minor tech powers thrown in for good measure.  He is one of the few squadmates in previous games who was proficient with the sniper rifle.

In Mass Effect 2 Garrus was a romance option for female Shephard (which provided some excellent commentary from Mordin, if pursued).

Vega is a new character to Mass Effect, having previously only appeared in an eight-page comic called Mass Effect: Conviction.  He's a soldier in the Alliance who has found himself on Omega after an unknown incident on Fehl.  In the comic he fights a group of Batarians and is then recruited by Captain Anderson for a special assignment.  The final panel indicates that this mission has something to do with Commander Shephard, who is apparently restrained.

It is unknown exactly what class Vega will be, but the comic seems to indicate he will be a regular soldier.

Vega will most likely be a romantic option for female Shephard, but that's unconfirmed.

A veteran of the first Mass Effect game, Kaidan is confirmed to be a returning squadmate in Mass Effect 3... provided you saved him on Virmire in the first game. Kaidan cameoed in Mass Effect 2, but because of Shephard's involvement with Cerberus he refused to join Shephard's squad. Sometime in between then and the events of Mass Effect 3 Kaidan has followed in Shephard's footsteps and become a council spectre.

In Mass Effect Kaidan was a biotic, and presumably that will be his powerset again in Mass Effect 3.

Kaidan was a romance option for female Shephard in the first game, and although interactions were limited in the second game, it is probable that such a romance can be rekindled or pursued again.

Liara has appeared in both Mass Effect and Mass Effect 2, as well as a 4-issue comic series from Dark Horse called Mass Effect: Redemption which bridged the gap between the two games, and serves as a lead-in to the Mass Effect 2 add-on adventure, Lair of the Shadowbroker. In Mass Effect Liara is a regular squadmate to Shephard after being rescued off the world Therum. Following Mass Effect Liara begins a hunt for the Shadowbroker -- an information broker referenced a few times in the first game -- which leads to her decision to not join Shephard's crew in Mass Effect 2, though she does appear in the game. Several months after the game was released a special mission with Liara was released in the Lair of the Shadowbroker expansion pack which temporarily lets her join your squad.

Liara, like most Asari, is a biotic and tends to use very powerful biotic skills in combat.

Liara was a romance option for both male and female Shephard (Asari, although visually female, are much closer to gender neutral) in Mass Effect. In Mass Effect 2 the romance could not originally be pursued, but in Lair of the Shadowbroker the romance could be rekindled, even if the player had chosen another squadmate to romance in Mass Effect 2 (Liara will make an amusing comment about whomever that is). What effect this will have on Mass Effect 3 romance options is as yet unknown.

Tali'Zorah vas Normandy, originally Tali'Zorah nar Rayya, is another squadmate who served with Shephard in both Mass Effect and Mass Effect 2. In the original game she was a quarian on her pilgrimage who came across a fragment of data useful to Shephard's attempt to unseat the turian spectre Saren, and then joined Shephard's squad to uncover the rest of Saren's plans. In Mass Effect 2 she leads her own squad of quarians across the galaxy hunting for information on the geth. She encounters Shephard twice during her hunt, and joins his squad after their second meeting despite having misgivings about the organization that's supporting them. She becomes an official member of the Normandy crew later in the game (a sign of her passing a rite of passage), changing her surnam to "vas Normandy" from "nar Rayya".  If she survives the final mission of Mass Effect 2 she will rejoin Shephard in Mass Effect 3.

Tali is a genius mechanic with lots of tech powers and an affinity for shotguns.

In Mass Effect Tali was just a squadmate of Shephard's, but in Mass Effect 2 she is a potential romance interest for male Shephard.

A "bonus" squad member gained by playing the "From the Ashes" DLC mission (available as a $10 download or part of the Collector's Edition), the Prothean has been in stasis for 50,000, waiting to defeat the Reapers. As the final surviving member of an experiment to fight the Reapers, Javik is a source of ancient, lost knowledge for the Normandy crew and Shephard. He's also extremely useful in a fight and can bring many formidable powers to bear.