iperf3: 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/iperf3 - from package: iperf3

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

On Debian possible default full paths are:

/usr/bin/iperf3 - from package: iperf3

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

Error in other languages:
iperf3: bevel nie gevind nie
iperf3: comandă negăsită
iperf3: comando não encontrado
iperf3: commande introuvable
iperf3: command not found
iperf3: käsku ei ole
iperf3: không tìm thấy lệnh
iperf3: komanda nerasta
iperf3: Kommando nicht gefunden.
iperf3: kommandot finns inte
iperf3: komut yok
iperf3: nie znaleziono polecenia
iperf3: níor aimsíodh an t-ordú
iperf3: no se encontró la orden
iperf3: no s'ha trobat l'ordre
iperf3: opdracht niet gevonden
iperf3: parancs nem található
iperf3: perintah tidak ditemukan
iperf3: príkaz nenájdený
iperf3: příkaz nenalezen
iperf3: команда не найдена
iperf3: командата не е открита
iperf3: 命令找不到
iperf3: コマンドが見つかりません