diff3: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/diff3 - from package: diffutils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/diff3 - from package: diffutils

You may install the required package with command apt-get install {package}

Error in other languages:
diff3: bevel nie gevind nie
diff3: comandă negăsită
diff3: comando não encontrado
diff3: commande introuvable
diff3: command not found
diff3: käsku ei ole
diff3: không tìm thấy lệnh
diff3: komanda nerasta
diff3: Kommando nicht gefunden.
diff3: kommandot finns inte
diff3: komut yok
diff3: nie znaleziono polecenia
diff3: níor aimsíodh an t-ordú
diff3: no se encontró la orden
diff3: no s'ha trobat l'ordre
diff3: opdracht niet gevonden
diff3: parancs nem található
diff3: perintah tidak ditemukan
diff3: príkaz nenájdený
diff3: příkaz nenalezen
diff3: команда не найдена
diff3: командата не е открита
diff3: 命令找不到
diff3: コマンドが見つかりません