ipython3: 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/ipython3 - from package: ipython3

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

Error in other languages:
ipython3: bevel nie gevind nie
ipython3: comandă negăsită
ipython3: comando não encontrado
ipython3: commande introuvable
ipython3: command not found
ipython3: käsku ei ole
ipython3: không tìm thấy lệnh
ipython3: komanda nerasta
ipython3: Kommando nicht gefunden.
ipython3: kommandot finns inte
ipython3: komut yok
ipython3: nie znaleziono polecenia
ipython3: níor aimsíodh an t-ordú
ipython3: no se encontró la orden
ipython3: no s'ha trobat l'ordre
ipython3: opdracht niet gevonden
ipython3: parancs nem található
ipython3: perintah tidak ditemukan
ipython3: príkaz nenájdený
ipython3: příkaz nenalezen
ipython3: команда не найдена
ipython3: командата не е открита
ipython3: 命令找不到
ipython3: コマンドが見つかりません