As has been stated before, building your own laptop is not something an average private consumer can physically do.
It requires special tools, experience with hardware, and even then the result could easily end up overheating with molten plastic being the direct result.
Bottom line, it's just not something you should even consider attempting.
------------------------------------------------------------------------
If you need recommendations for a place to buy a laptop that could handle BF3, I'd need a few pieces of information first, after which I can do my best to find a place, with varying results based on your geographical location.
#1:
Which country do you live in?
#2:
Do you need a laptop that can play the game at minimum settings, or do you need something that can actually make it look semi-good?
#3:
What kind of sizes are we talking here? Keep in mind that a laptop capable of running the Frostbite 2 engine will also generate a lot of heat, and heat couples with small outer shell means you either need a jet engine to cool it, or it will get hotter than hell and damnation.
------------------------------------------------------------------------
Now, with that out of the way.
Death_Korps_Kommissar said:
Ah but you see my desk top is a MAC os :/
This is just another reason why I'd never buy a desktop from Apple.
I'm loving my iPhone, and I can see the reasoning behind buying a laptop off them, but a desktop should be thoroughly upgradeable, and it's my understanding that Macs... well; just aren't.
But that's not helping you now, so no need to delve further into that cave.
------------------------------------------------------------------------
So yeah.
The bottom line is that a decent gaming laptop that won't die on you or give you third-degree burns the length of your arm, are costly.
As it stands, you could probably buy a study-laptop that can't run games, and then build a decent desktop, for less, which is what I did when I was in your situation three years ago.
The last resort is indeed a Laptop and Console combination, but personally I'm on the "Consoles are inferior" bandwagon, so while it could be the only option in your price range, I can't recommend it.
Besides, then you'd probably need a TV as well?