How to get Photoshop CS3? I guess you want stand alone Photoshop edition, right?
I think, stand alone version of Photoshop is still available to download. I believe it's version CS6 and costs a lot of money.
Does $700 sounds right?
If you still have your Photoshop CS3 (Windows version) on some media there is another way.
Did you know you can install Windows on Mac? Then you can switch between OS and Windows.
If you want to have Photoshop CS3, the only way is to install Windows on your Mac and then install Photoshop, like
you did on your old computer. If this is too much hassle for you, I'm afraid there is no other legal way to get CS3 on your Mac.
The other way is to buy still available stand alone CS6 version for Mac.
The third way is to go for subscription - Photoshop version CC, I believe.