Your problem is that &
is used to separate multiple flashvars variables. So, your flash get 4 values instead of 2. The solution is to urlencode your url. It would give you the following embed :
<embed allowScriptAccess="always" wmode="transparent"
src="http://abcpage/A.swf"
flashvars="clickTAG=http%3A%2F%2Fexample.com%2Fabcpage%3Fvar1%3D123%26var2%3D223%26var3%3D224&clickTARGET=_blank" />
And that's all !