Xcopy change source folder if doesn't exist

I don't have any problem to do Xcopy, but the source folder may be not the one specified. It could be one among three folders.

So I would like a proceed that

set sourcefolder = "folder1"
if not exist set sourcefolder = "folder2"
if not exist set sourcefolder = "folder3"
if not exist exit

But I'm a newbie in scripting, so I would be glad if anyone could help me please.