These three missions are pretty easy. Some people have said that Goalpost is one of the hardest, but in reality, I died twice on the turret sequence.
First, Mind the Gap. First part is sneaking. Second part is a firefight at the docks. Just try to move up shoot when necessary, your team can take care of them. After opening the truck, move to the loading platform kill the guys that come from around the corner. The heli will take care of anyone on the upper level. For the construction yard, just move up using cover, your teammates are your greatest weapon on veteran. For the truck sequence, just shoot at the train. For the train station, there will be a long hall. Thin out as many enemies as you can, toss a flash, move up. When you get the end of the hall kill the guy that comes out to the right. At the bottom of the escalators there are two or three guys to the right one on the left and one far left. Kill left to right, reinforcements will come on the left side. toss a flash if necessary then kill the guys to the right. Move up the escalator and use it for cover at the top. Kill then repeat for the next escalator. Mission done.
For Goalpost, move up using the rock for cover. When you get to the beach wall, move toward the crashed plane on the right side. Kill the guys behind the boxes. Now just move up the street when you can staying on the right side. Between the tanks and your squadmates you wont have to stick your head out much. When you get on the tank's minigun, focus your fire on the right side while periodically checking the middle and left. When in the parking garage, shoot the cars to kill the enemies. There is a part where the tank crashes through a small office, as that happens kill the guys on the other side. After the tank sequence, you are out on the street again. Replace your useless rocket for the scoped M14 nearby. This gun will help when your team is moving through the buildings. Just keep trying to move up, flashing the enemy if necessary. After you exit the building move the opposite side of the street. Thin the enemy out then move up to the bus. At this point your squad should move to the end of the road to the turn. At the turn there will be a spot for cover near the left wall. Stay there until you guys kill everyone. The last part is the breaching and that's it.
Return to Sender is the easiest out of these. For the beginning you have the chopper gunner at your disposal. Move to the left side as there is a Draganov if you want it. Thin out as many guys with the chopper gunner, then move to the L shaped building. Eliminate the three guys on the upper level. Then chopper gunner the rest. When you move inside the target building you will engage guys on the second floor. Use two or three grenade launcher rounds to take care of them. Breach the door and kill right to left, there should be 5 guys(Two right, two mid-left, one left). When you need to get to the chopper move to the left side and move up when you can. When it is clear, stay with Soap. He will move to a house on the right. Take out the guys on the roof and the dude on the second floor from here. Move up and take the guy in front of the house then the guy on the left. Next part is tricky. Move to the stair next to the incline. At the top there will be a guy on the roof in cover behind a sign. Kill him. Then move to where your team is. Toss a flash them move to the hut. Kill the guy around the corner then there should be guys in the house and the a dude behind the box next to the house. Flash it, tube it, Flash again, move up kill the guy behind the box, run. There will be a building that you need to get to the top of, DONT GO IN IT YET. Instead move to the right side and toss grenades into it when guys come out. Move all the way up the side until you get to the wall. Price should say all clear. Move to the second level and kill the 3 dudes there. Toss a flash if you need to. When at the top, chopper gunner. You should be in a dust storm now. Ready your grenade launcher. You should have 4 or 5 rounds. Every time you encounter enemies grenade them. Move to Nikolai and go prone. When its time to move him, pick him up and don't stop. Fall down the hill, run to the Jeep, done.
Adapt when necessary. This is just what I did when I did this.