Just for safety concern, someone should measure voltages coming out of their A/V cable lines. Stereo headphone plugs are only 3 contacts (tip and 2 barrel contacts). The Canon A/V cable has 4 contacts on the plug barrel (end and 3 barrel contact zones). It would be nasty if some important contacts were grounded together for any length of time inside the camera and might cause some problems.
Actually, the cable that came with my a570is has a standard 3,5 mm stereo plug in one end and two RCA plugs in the other end. Video and mono audio (tip=audio, ring=video, sleeve=gnd common to both). Does the s3 have stereo sound?
Anyway, any decent equipment shouldn't be harmed by shorting this kind of plug because the plugs are defective by design in that sense (even the right plug for the right socket shorts things out while plugging or unplugging, and when not pushed far enough the condition may be remain for undetermined periods of time). I'm not saying it's particularly smart to short things out, though