#!/bin/bash
SOH=$'\001'
STX=$'\002'

for f in File{1..3}.cpp; do
    sed "s/^/$f: /" $f
    echo
done

echo File1.cpp with STXs for LFs:
<File1.cpp tr \\n $STX | cat -A
echo; echo

echo "Replacement made:"

<File3.cpp tr \\n $STX \
  | sed "s$SOH$(tr \\n $STX <File1.cpp)$SOH$(tr \\n $STX <File2.cpp)$SOH" \
  | cat -A

echo -e "\n\nFinal result, STXes converted back to LFs:"

<File3.cpp tr \\n $STX \
  | sed "s$SOH$(tr \\n $STX <File1.cpp)$SOH$(tr \\n $STX <File2.cpp)$SOH" \
  | tr $STX \\n