$ paste -d "|" filex filey filez | awk '
> BEGIN{OFS=FS="|"} {print $1,$2,$4,$6,$2+$4-$6}
> ' | awk 'BEGIN{OFS=FS="|"}{ for(i=0;i<=NF;i++)
> if ( $i == "" )
> $i=0
> print $0}'

<Output>
1|100|200|300|0
2|200|200|100|300
3|300|600|200|700
4|400|800|0|1200
5|500|0|0|500