No, getting a license for CS6 does not give you a license that will allow you to install and activate any earlier version!
Thanks for getting back to me so quickly!
That being the case, would you happen to know how to get hold of a copy of CS3-5? The best I can find is dodgy-looking copies on ebay... Do Adobe still even sell these earlier versions? Or is there some workaround that will enable me to work on CS3 documents in CS6?
Adobe stop selling old versions the day the new one comes out. Dodgy old versions are all there is. Counterfeits abound. Not to mention it's going to be a problem to install and run on up-to-date systems. You have a problem with accepting this work...
Sorry missed one point. You can open in CS6 but it's a one way process. Realistically teo people collaborating need the same version. Cs6 is, by the way, also superseded, hard to buy, and out of support, but a CC subscription lets you run it.