An Archive and Install
should preserve third party software, but I've heard Adobe can be tricky.
As for putting it on an external and then moving it back, Adobe CS is known to be very fussy about getting moved. It might think you are trying to install it on another computer and prevent you from moving it to the external and then back. But anyway, since it has files all over the place, I don't think it's as simple as moving the application over. You will probably need to do a clone -- a clone will bring everything over -- which might get around the license issue, and then reverse clone from the external back to the Mac. But on the CCC Forum, I have read that some people have had problems cloning Adobe, not sure which version though.
If you have the CS3 Install Disc, then you might just want to de-activate it first and then reinstall once you've done the system reinstall.
One excellent cloning program is Carbon Copy Cloner
http://www.bombich.com/
And, if needed (according to this, you are allowed to install it on two computers, so maybe it isn't necessary to deactivate it first. Then, again by deactivating, you would preserve the option to install it on another computer):
http://www.mydigitallife.info/2008/07/17/how-to-deactivate-and-reactivate-adobe- photoshop-cs3-to-transfer-or-move-activation-license-key/
Message was edited by: WZZZ