I see all the answers as either asking to compile python3 from code or installing the binary RPM package. Here is another answer to enable EPEL (Extra Packages for Enterprise Linux) and then install python using yum. Steps for RHEL 7.5 (Maipo)
yum install wget –y
wget https://dl.fedoraproject.org/pub/epel/7/x86_64/Packages/e/epel-release-7-XX.noarch.rpm # Verify actual RPM name by browsing dir over browser
rpm –ivh epel-*.rpm
yum install python36
Also see link