My goodness! 3 cartons a week is way too much! Seriously.
I'm going to give an answer though as I assume you are young, at college and have mates drinking your beer as well i.e., it's not just you drinking all that beer. Or, maybe bottling is driving you to drink which is quite understandable.
Also, there is plenty of information on AHB re keg set-ups so I'll throw a few links in at the end of this post which will help you get your info a lot quicker. They are new threads but reading them will answer the question you have now and many others. Reading these threads will also enable you to source everything I have listed below at close to the prices listed.
Cheapest and bare minimum keg set up you will need is...
1 x 19 litre cornelius keg $80?
1 x Bronco Faucet & Hose $15
1 x Gas Regulator $80-$95 (Harris or Micromatic brands)
1 x D Size Gas Bottle $70? Initial cost plus $35 for re-fills (monthly in your case - just joking!)
1 x 25 litre 'cube' $20
2 x Disconnects & Lines $30
The 2 links that will help you with not only sourcing the above but in making quality beer are...
Outline To Using The Ahb Website
Frequently Asked Questions For The New Brewer
Have a good read of the links above and let us know how you go with your kegging set-up.
Cheers
PP