Expression needed

Hi there

I need an expression that enables three continuously changing values to always add up to 1.

Basically, I have the swatch object, and want to be able to do a sweep through the colours with the slider object. I have the pack object going into the swatch object and know I need three float values going into it for the RGB values but this is as far as I've got!

Is there a simple way to do this?

Many thanks,
Jessica.