Are all SATA cables compatible with SATA 3?

Yes. There is no difference in the cables. They have not changed with SATA revisions.

When the Serial ATA International Organization (SATA-IO) began design of the SATA Revision 3.0 specification, the designers wanted to double the interface data rate while enhancing support for multimedia applications − without introducing major changes that could adversely affect cost, increase design complexity, or reduce the quality and performance of the SATA interface. Key to meeting this goal is maintaining backward compatibility with SATA 3Gb/s connectors and cabling.

Source (pdf): http://www.serialata.org/documents/SATA-6-Gbs-The-Path-from-3gbs-to-6gbs.pdf

We were discussing this on chat - I did a little digging and both maximum PC and puget systems did benchmarks on various cables.

Maximum PC had a more interesting variety of cables and found they worked identically, even when used beyond the specified distance. Puget system tried it with a typical set of cables and found there's little difference.

Empirically, if its a to spec, non damaged cable, it should work fine even with sata 3. Modern cables are probably made to higher tolerances and should work alright. I'd go with 'sata 3' cables if I had them but for most purposes, It looks like it dosen't matter.