Why would you limit your laptop choice when there may be other feature just as important like memory or processor speeds.
I would just use a external USB or bridge. In most cases these actually work better because many have external antenna. Another advantage is you do not have to worry about how your laptop screen is positioned (the antennas are normally behind the screen).
All that said 450m is another magic marketing number. You will only get that in a lab condition and in many cases it will actually make your connection slower. Since to get 450m they use the complete 2.4g range. This means if anyone else near you has a wireless router they are 100% guaranteed to interfere with you. Your router will try to fix this by changing the speeds and encoding on the fly but this changing is a common cause of dropping and authentication delays.
I am looking to buy a laptop with three antennas capable of 450 Mbps speed.
The only one I know of is the Alienware 17 and 18 inch, which have the Killer 1103 wireless card pre-installed and 3 antennas.
I think the new MacBook Pro has an Intel Centrino Ultimate Wireless-N 6300.
Are there any others that anyone knows about?
There was a Dell E5210 but it is discontinued.