It seems the current script can also be used to install v3 but you need to supply the desired version as a parameter- "Please provide the desired version. e.g. --version v2.4.0 or -v latest"
Otherwise it seems to default to v2.16.1, thanks to bacongobbler's nice comments
# checkDesiredVersion checks if the desired version is available.
if [ "x$DESIRED_VERSION" == "x" ]; then
# FIXME(bacongobbler): hard code the desired version for the time being.
# A better fix would be to filter for Helm 2 release pages.