I think the only solution is to get an RGB-capable SCART switchbox. Making the two other SCART's on the set capable of accepting RGB would require more or less heavy modifying of the sets internal electronics.
How many things have you got to plug in then.......
Can you not just daisy chain them together........??
I have only got 1 RGB scart on mine, but i have 3 things going to that.
Scart 2 should be S-Vid and is not far behind RGB..............infact i prefer the pastel colours of S-Vid to the bright RGB colour and if you have a A/V amp, you will be able to switch most stuff via that.
I have a further 4 things going through the amp on S-Vid and the VCR on Scart 3 S-Vid.