Note - I don't use Kaleidoscope anymore, seems to be abandonware

Using custom diff tools in Rational Team Concert (RTC) is a bit tricky, especially when that tool is Kaleidoscope. But there is a way to do it. Here's a screenshot of my RTC Preferences:

The script ksdiff_rtc is used because Kaleidoscope does not allow a three-way compare with the base and local copies to be the same - but that's how RTC rolls. So it creates a temporary copy of the local version of your file in the /tmp directory:

/usr/local/bin/ksdiff_rtc:

#!/bin/bash

filename=$(basename $3)  
cp $3 /tmp/$filename  
/usr/local/bin/ksdiff --merge --wait --output $1 --base $2 -- /tmp/$filename $4 --snapshot
rc=$?  
exit $rc  

Hope that helps!