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This can look backwards
awk '{seen[$2]=$3}
($1 in seen) {print $1,seen[$1]}' input

Typically a rough main structure would be:
while read var1 var2 var3
do
# do something with $var1 $var2 and $var3, for example
echo "$var1, $var2 and $var3"
done < infile > outfile

You mean output like
4 12
5 13
??

Try:
awk '$1==$2{print $1" "$3}' input