Your post is somewhat ambiguous as it makes reference to three machines.
I assume you are "scrapping" the old XP machine. I take it that you mean you have a laptop for which you have a Win8 licence.
As I understand it, under EU law, you may transfer that licence to a new machine, but, of course you must remove it from the original PC first.
If you do not have the 25 character code for your installation, then you will need something like Recover Keys to find it from the installation.
However, unless you have a touch-screen computer, I would not go for WIN8. WIN7 licences are readily available on e-bay (or they were prior to April 8th!!!!) for around £45 and will give you the COA you need.
The only other thing is to make sure you have the drivers for the various items of hardware, or know what they are to download the drivers BEFORE starting! Note that usually the same driver will do both WIN7 and WIN8, but do make sure you have the correct installation media (32 or 64 bit). The COA will licence either version