ppm3d: 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/ppm3d - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppm3d - from package: netpbm

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

Error in other languages:
ppm3d: bevel nie gevind nie
ppm3d: comandă negăsită
ppm3d: comando não encontrado
ppm3d: commande introuvable
ppm3d: command not found
ppm3d: käsku ei ole
ppm3d: không tìm thấy lệnh
ppm3d: komanda nerasta
ppm3d: Kommando nicht gefunden.
ppm3d: kommandot finns inte
ppm3d: komut yok
ppm3d: nie znaleziono polecenia
ppm3d: níor aimsíodh an t-ordú
ppm3d: no se encontró la orden
ppm3d: no s'ha trobat l'ordre
ppm3d: opdracht niet gevonden
ppm3d: parancs nem található
ppm3d: perintah tidak ditemukan
ppm3d: príkaz nenájdený
ppm3d: příkaz nenalezen
ppm3d: команда не найдена
ppm3d: командата не е открита
ppm3d: 命令找不到
ppm3d: コマンドが見つかりません