Presumably all three skin file have a common CSS too.
If you put them in the same directory, you can use a single CSS file, with additions as necessary.
If there are major parts of the three skin files (header and footer), I would recommend pulling that part out into separate files that can be included in the skin files. You can include lines like this in your skin file:
<!-- #Include virtual="Skin_Header.txt" -->
That way, you only have one copy of the header, footer or whatever. Updating is much easier.