Fibber, many of the CFS3 sound packs use the stock supporting wav files for crashes, ground roll, wind, landing touchdowns and other miscellany located in the main CFS3 SOUNDS folder. So you first have to copy all the contents of your CFS3 sounds folder to a temp location, then copy the contents of your CFS2 main sounds folder to the same temp, choosing any overwrite if prompted. That gives the CFS2 files top priority by ensuring that any CFS3 files of the same names will not overwrite. After that you can use any CFS3 sound pack by just changing the header in the cfg from:
[CFS3]
product_code=CFS3
>>>>----------to--------->>>>
[FLTSIM]
product_code=FSIM
This [FLTSIM] header is compatible with FS98, FS2000, CFS1, CFS2, FS2002, FS2004 and FSX. Why CFS3 needed its own sound code within the MSFS line is beyond me.