python3: 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/bin/python3 - from package: python3-minimal

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

Error in other languages:
python3: bevel nie gevind nie
python3: comandă negăsită
python3: comando não encontrado
python3: commande introuvable
python3: command not found
python3: käsku ei ole
python3: không tìm thấy lệnh
python3: komanda nerasta
python3: Kommando nicht gefunden.
python3: kommandot finns inte
python3: komut yok
python3: nie znaleziono polecenia
python3: níor aimsíodh an t-ordú
python3: no se encontró la orden
python3: no s'ha trobat l'ordre
python3: opdracht niet gevonden
python3: parancs nem található
python3: perintah tidak ditemukan
python3: príkaz nenájdený
python3: příkaz nenalezen
python3: команда не найдена
python3: командата не е открита
python3: 命令找不到
python3: コマンドが見つかりません