nagios3: 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 Debian possible default full paths are:

/usr/sbin/nagios3 - from package: nagios3-core
/usr/lib/debug/usr/sbin/nagios3 - from package: nagios3-dbg

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

Error in other languages:
nagios3: bevel nie gevind nie
nagios3: comandă negăsită
nagios3: comando não encontrado
nagios3: commande introuvable
nagios3: command not found
nagios3: käsku ei ole
nagios3: không tìm thấy lệnh
nagios3: komanda nerasta
nagios3: Kommando nicht gefunden.
nagios3: kommandot finns inte
nagios3: komut yok
nagios3: nie znaleziono polecenia
nagios3: níor aimsíodh an t-ordú
nagios3: no se encontró la orden
nagios3: no s'ha trobat l'ordre
nagios3: opdracht niet gevonden
nagios3: parancs nem található
nagios3: perintah tidak ditemukan
nagios3: príkaz nenájdený
nagios3: příkaz nenalezen
nagios3: команда не найдена
nagios3: командата не е открита
nagios3: 命令找不到
nagios3: コマンドが見つかりません