There is pretty much no way you and client can work together. At this point the only option is for the client to upgrade by buying CS6 or subscribing to Creative Cloud.
Neither CS3 nor Version 3 (which would be CS) should be used now anymore and there is no direct way between any modern version.
Would it be CS3, you would have to export as IDML, open that in CS4, export there as INX and open it in CS3.
If it would be version 3, you would have to export as IDML, open that in CS4, export there as INX and open it in CS3, export there again as INX, open that INX in CS2, export again as INX, and open it in CS.
But every down converting will cause change and damage to your document. It will be useless at the end.