Import the RPM:
rpm -ivh http://repo.zabbix.com/zabbix/3.4/rhel/7/x86_64/zabbix-release-3.4-2.el7.noarch.rpm
Further RPMS can be located here. Esnure that you use a RMP version that aligns with your Zabbix release version.
yum install the zabbix-agent package:
yum install zabbix-agent
Start the zabbix-agent service:
systemctl start zabbix-agent
Enable auto start of the zabbix-agent service:
systemctl enable zabbix-agent