To install iperf3 on CentOS 7 in offline mode, you need to download two files one for EPEL libraries and one for iperf3, upload to your CentOS machine with WinSCP and then follow two command (given below) to install the iperf3.
Click here to download the latest version of iperf3
Click here to download the latest EPEL library file
EPEL libraries file should have name like this epel-release-*.rpm
Upload both files to your CentOS machine, say in /tmp folder
Take SSH Session of CentOS Machines
Go to the folder where you uploaded both files, i.e. /tmp
Install EPEL release rpm with following command
# rpm -Uvh epel-release-*rpm
Install iperf3 with following command
#rpm -Uvh iperf3-*.rpm
iperf3 has been successfully installed on your CentOS machine.