I bought the battle chest and i have the game and extension pack. i have a computer and im getting a new laptop for christmas. can i download it with the disk on my computer and then on my laptop at christmas or will it not work twice?

It will work twice. It's linked to your battle.net account. So just as long as you log into that on your laptop at christmas you'll be just fine.

My disks have been used on 2 different computers and a laptop now.

All you are REALLY paying for are the codes that get tied to your account. You are the only one who can use those codes, but anyone can use the disks.

Yes you can use the discs on any PC you may purchase in the future. The game files are actually free, you're paying for the registration code.