If pip did install iperf3, it should be in /usr/lib/ directory.
Maybe you just need to create a symlink to libiperf.so.0.
Maybe it was supposed to be iperf, not iperf3.
MIPS has a lite version of guestshell.
you can try 'pip install --upgrade iperf3' to see if it will update.
Let us know what you found.