You can't use sed because its processing is line-based. You can use awk if you unset the record separator, like this...

awk ' BEGIN { RS="" }
      FILENAME==ARGV[1] { s=$0 }
      FILENAME==ARGV[2] { r=$0 }
      FILENAME==ARGV[3] { sub(s,r) ; print }
    ' exfile2 exfile3 exfile1 > exfile4

asdasdasdasd
asdasdasdasd
jkl
mno
pqr
sdasdasdasda
asdasdasdada