Your squad in ME3 consists of:
The Virmire Survivor, which means one of Ashley or Kaidan, as well as the return of Garrus, Tali, and Liara. In addition, two new members join the squad, James, and a newly corporeal EDI. If you own the From Ashes DLC, there is also Javik, the Prothean.
As for 'classes', the squad members don't have classes in the sense that Shepard does. Instead they each have their own unique set of 4 powers. Two are shared with one of the available Shepard classes, and two are unique to them, but can be unlocked as Bonus Powers through conversation.
Specifically, the lineup of powers for each looks something like this:
Ashley Williams
Disruptor Ammo Concussive Shot Inferno Grenade Marksman
Kaidan Alenko
Reave Barrier Overload Cryo Blast
James Vega
Incendiary Ammo Carnage Fortification Frag Grenade
Garrus Vakarian
Concussive Shot Armor-Piercing Ammo Overload Proximity Mine
Liara T'Soni
Singularity Warp Stasis Warp Ammo
Tali'Zorah vas Normandy
Defense Drone Energy Drain Combat Drone Sabotage
EDI
Incinerate Overload Defense Matrix Decoy
Javik
Dark Channel Lift Grenade Pull Slam
Be warned; spoilers follow...
Kaidan or Ash shows up at the end of the first mission. They will be unavailable to the party for a while. They can possibly die, or re-join the party later on.
Liara joins the party on Mars.
Garrus shows up when you go to rescue the Turian leader and joins
your party
Kasumi shows up when you agree to help a fellow Specter investigate a Hanar diplomat on the Citadel. When the mission ends, she can join up as a war asset.
Thane is in the hospital on the Citadel. He cannot join your mission since his condition is terminal.
Miranda will communicate with you several times, and meet up on the citadel. You will later encounter her on a mission where it is possible for her to die. If she survives she will become a war asset.
Zaeed can be found on the Citadel while investigating the Volus
ambassador. This quest is triggered after the Cerberus attack on the
Citadel. He can join up as a war asset.
Wrex, shows up at the summit between the alien leaders after you
retrieve the Turian patriarch. He stays on the Normandy, but he does
not join your party.
Mordin can be found on the mission to rescue the Krogan Females. He
will come aboard the Normandy, but he won't join your party.
Grunt is found on a mission from Wrex to rescue some Krogan scouts.
He can join up as a war asset.
Jack shows up during the rescue mission to the Grissom Academy. She
may join up as a war asset, and you can later talk to her at the
night club on the Citadel.
Jacob is found in the mission to retrieve some Cerberus scientist
that is triggered after the attack on the Citadel. He can join up as
a war asset and can be talked to at the Hospital.
Samara is found in the mission to investigate an Asari colony that
is triggered after the attack on the Citadel. She can join up as a
war asset and can be talked on the Citadel
Tali and Legion are both found in the mission to recruit the Quarian
fleet. Tali can join your team, but what happens to Legion is very
dependent on player choice.
During the end game you will have the opportunity to speak with all
your surviving squad mates past and present one last time.
Some other Normandy crew non-squad mates show up as well.
Dr. Chakwas can be found in the Citadel hospital and can re-join the
crew.
The two Normandy engineers from ME2 can be pardoned in the Spectre
office, and will rejoin the crew.
Kelly Chambers can be found in the refugee camp on the Citadel. She
won't join your crew. You should also convince her to go into hiding
or she'll get assassinated by Cerberus operatives when they attack
the Citadel. She can also be romanced.
At the end of the game you will be able to communicate with all of your surviving squad mates before the final mission.
Here's some information to help with keeping your team mates alive.
Best Answer
As FAE pointed out, this is a known bug on the Bioware forums. As documented there, this problem appears to be a bug caused by the "new" tag to not be removed properly from one of the quests. That means that choosing "mark all as read" should fix the problem. I tested this and now the mission screen starts by showing the top of the list as it should.
EDIT: It looks like this fix only works for a very short period of time, and it is not repeatable. A simpler but less effective fix, as mentioned in the linked forum post, is to simply sort by name every time you open the mission screen.