If you didn't get the mission from McCarran, go back and try again.
The Major who gives out the missions usually hangs out in front of the terminal or on the road leading up to it.
If he's not there, walk around from one side of the tent area to the other
and you might either find him or he will eventually go to his usual spots.
Exit McCarran and continue straight to the highway, then turn left.
If you got the Classic Inspiration mission, take a picture of McCarran
At the bridge, cross over the highway and turn right, heading to a fence of
barriers running E-W. Get a picture of the Sunset Sarsaparilla bottle statue .
Turn left and go down the full length of the barriers/fence and turn left.
You will be looking at Cook-Cook's building and beyond that and to the right, an old gas station, and past that, Violet's compound.
Save. Kneel down and fire.
When the action dies down, loot bodies and kill any stragglers. When you get to Cook-Cook's body, if his head isn't intact, load from your last save and tell Boone to watch where he's shooting and try again.
(I recently bought a Missile Launcher at McCarrans and fired a missile at Cook-Cook, fully expecting his head to be blown off, but to my surprise, it wasn't.)
Once you have Cook-Cook's head, go south to Violet's area. After you kill Violet, go through a break in the corner of her chain link fence. You can see a door to the Fiend area with barrels burning on either side. You will need Lockpick=75 once inside. If you don't have it, you can get a key from Anders as described below.
Cook-Cook's building is closer to the entrance at top, but the entrance at lower left is closer to Vault 3 once you get inside the Fiend's area. On the other hand, you get XP for "discovering" the entry at the top but not for the one at the bottom. The two entrances look alike except that the top one has dumpsters on either side of the doors.
If you come in the lower entrance, you will see the building shown below.
A doorway into the building is at the far left corner. Go in and continue the length of the building.
Once V3 is registered on your map, before entering you can fast travel to unload loot
and to sleep for the Well Rested bonus. Save before entering the Vault.
A hostile fiend will ask what you want. With Speech of 64+, you can get invited in as
a friend; otherwise, about 8 Fiends will attack. Lie and things are much easier.
Follow the map below to Motor-Runner's area. It's a short trip but another 8 or so fiends will attack.
After you get through the hard-locked door into M-R's area, there is another locked door on the left.
Check the lockers in that room for vault utility suits which give +5 to Lockpick and +5 to Repair.
In the Living Quarters area there are more suits in lockers. Use them to repair one to keep.
You can also use plain Vault 3 jumpsuits to repair the utility suits.
The game indicates that you'll be paid if you kill Motor-Runner in Vault 3, but I got nada.
The actual mission is to save Anders who was sent before you and work out how to kill Motor-Runner.
However, when I reported to Hsu without finding Anders, he says thanks for saving Anders and killing M-R, but still no money or XP. I've read that you have to find Anders before killing Motor-Runner.
I've tried finding Anders first and it made no difference except that you get some options from talking to him. If you tell Anders to help you kill M-R, you can get some XP during your talk, but after he jumps up ready to go, tell him that he should head back and he will.
When you first come in and kill some fiends, if you go up some stairs and immediately look left, you'll see an open door to a small room (left of the yellow star in the middle map. If you were to go through that door, you would run into a smooth panel which Anders can open from his side. If you keep track of Anders' position, it's easy to get to him.
To find Anders, follow the middle map above: Go up the stairs and turn left at the first hallway where you see "Keep Out" painted on the wall. Go to where stairs down are on the left. Turn left at the bottom of the stairs and go to the stairs going up to Anders' room. There is a trip wire and a grenade boquet after that and some land mines at the top of the stairs, then you get to Anders.
You don't have to "rescue" him. He is just sitting on the floor and when you tell him to go back, he opens the panel door to the vault door area and he's out. He could have left at any time.
The third map above, instead of turning right when you come in, turn left and go until you enter this wing, then follow the line to the Overseer's office. This leads to finding a stash of loot in a flooded area. If you don't have the underwater breather from the Boomers, you can come back later, but it's not worth it.
Use the first map above to get from Vault 3 back to the Fiend area's entry, exit, turn left, and get the last Fiend (Dephi) for the Major.
Fast travel to the Lucky 38 Presidential Suite to get some sleep for bonus XP, then
return to McCarran and turn in the heads/missions to the Major.
See Col. Hsu to tell him you killed Motor-Runner, not that it gets you anything.
Before entering Vault 3, if your backpack is pretty full you can fast travel to Gun Runners to sell stuff then FT back to Vault 3 at which point there will be some fiends to greet you. Kill them and then enter the vault. Save before entering the vault in case something goes wrong.
I maxed out Boone's carry capacity and gave ED-E a lot too. When I went to Gun Runners, ED-E repaired everything to 100% and I ended up with 40,000 caps.