I would agree with the general concensus that desktop PCs are a more robust solution for teenagers.
A few suggestions:
If you buy 3 identical PCs, you can save yourself a good deal of time and effort by also investing in Ghost or similar disk cloning software. Not only can you set all of them up the same (locked down ;-) ) but storing an "as first built" image is a great way to cure problems later! You will need to obtain SYSprep for MS to do this properly.
My children are younger than yours (9 & 6), so they are lowly users - don't give admin rights unless your children are responsible, PC literate young adults! If that's not a contradiction in terms! (Actually I had to downgrade my wife to User-only, after she downloaded a dodgy screen-saver
).
Work out in advance how you want to "Police" internet usage - as well as being able to block domains in your firewall you might consider Net Nanny type of software.
Consider some form of anti-spam product. And ensure that MS updates are applied regularly (I don't advocate autoupdate - blind faith in MS... no thanks!)
Antivirus is critically important - make sure it is maintained up to date (auto-update if possible), has on-the-fly protection and schedule a daily sweep. Don't let it be disabled!
One really useful piece of software I have is Virtual CD - saves having stacks of CDs being popped in and out for each and every game.
Perhaps most important is to educate your children (as long as you can stay one step ahead!) so that they become responsible netizens and computer users.
I am convinced that in the not-too-distant future it will be necessary to have insurance and even take a test to operate an internet-connected PC. As society becomes ever more dependent on the internet for communications, e-commerce and "identity management", people will be sued for negligently allowing unprotected PCs to be used as zombies for cybercrime. A bit like drink-driving, or knowingly passing on HIV.
Maybe I'm paranoid...
SD