If you have a PC you don't want the Mac version -- it won't install. And the only way you'll find a version of CS3 now is to scour the auctions and odd-lot distributors, but it's pretty risky.
You can export a .inx file from CS4 that they should be able to open, but it's not a great workflow. Text will probably reflow, and if you mistakenly use any unsupported features they will be lost in the conversion.
I have a Mac Computer too, for this job. I need to be able to do layout, so I can't just send them pdf files. (for a two page insert into the daily paper). I need to be able to exchange files regularly.
so you're saying Adobe has no way of selling me CS3 - I can only try to get someone's old version? I've found someone, but they are unsure about licensing issues...
First, education versions cannot be transferred. Second, any version that is used as part of an upgrade chain cannot be transferred without the rest of the chain, so if your friend has an extra standalone license, or an upgrade license they no longer want and are willing to give up the whole chain (including newer versions that were upgraded from this one), then you've got a potential solution. You have to contact Customer Service for a transfer form (it might be available online, if you search) that they will have to fill out, sign, and mail in so you can register.