I saw this posting which explained how to get BC3 working as the diff tool for Subversion... but what about using Beyond Compare 3 to do 3-way merge/compares?
Here is a linux version of liamf's script that works with svn 1.6.
#!/bin/bash
MYTITLE=$4
OLDTITLE=$6
YOURTITLE=$8
MINE=$9
OLDER=${10}
YOURS=${11}
OUTPUTFILE=${MINE}.merge
/usr/bin/bcompare -solo -automerge -force -reviewconflicts -favorleft -lefttitle=$MYTITLE -centertitle=$OLDTITLE -righttitle=$YOURTITLE -outputtitle=$OUTPUTFILE $MINE $YOURS $OLDER $OUTPUTFILE
RESULT=$?
if [ $RESULT -eq 0 ] ; then
cat $OUTPUTFILE
exit 0
else
exit 1
fi