open the document in the cs5 and export the file in "IDML"
Open this "IDML" file in cs4 nad export it again in "INX"
this INX file will be compatible and will open up in CS3 ..............You need to have CS4 for this.
What Manish said, but keep in mind that it may not look the same in CS3 as it did in CS5. Some editing may be required.