the key issue is not whether a swf is created with cs3 or flash 8. it's whether the swf is published for as2 or as3.
an as3 swf can load as2 swfs and both the actionscript 2 and 3 will work.
an as2 swf can load an as3 swf but the actionscript in the as3 file won't work. however, you can circumvent that limitation by creating a new master as3 swf.
Thanks. What code will i use in my AS3 master swf.
use the loader class:
var loader:Loader=new Loader();
Is that in the frame? or On a button?
all code should be attached to a frame. never attach code to a button under any circumstance (unless you're publishing for as1).
in fact, in as3 you cannot ever attach code to an object. in as2 you can but should not.