kdiff3: 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/kdiff3 - from package: kdiff3

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

On Debian possible default full paths are:

/usr/bin/kdiff3 - from package: kdiff3-qt
/usr/bin/kdiff3 - from package: kdiff3

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

Error in other languages:
kdiff3: bevel nie gevind nie
kdiff3: comandă negăsită
kdiff3: comando não encontrado
kdiff3: commande introuvable
kdiff3: command not found
kdiff3: käsku ei ole
kdiff3: không tìm thấy lệnh
kdiff3: komanda nerasta
kdiff3: Kommando nicht gefunden.
kdiff3: kommandot finns inte
kdiff3: komut yok
kdiff3: nie znaleziono polecenia
kdiff3: níor aimsíodh an t-ordú
kdiff3: no se encontró la orden
kdiff3: no s'ha trobat l'ordre
kdiff3: opdracht niet gevonden
kdiff3: parancs nem található
kdiff3: perintah tidak ditemukan
kdiff3: príkaz nenájdený
kdiff3: příkaz nenalezen
kdiff3: команда не найдена
kdiff3: командата не е открита
kdiff3: 命令找不到
kdiff3: コマンドが見つかりません