We have a SAMBA share on a centos 6 machine.
Question: Can we mount this SAMBA 3 share over the internet? (without VPN/SSH tunnel, so directly!)
It depends on some aspects of the protocols and implementations. NetBIOS/NetBEUI is not routable at all and it works sending broadcasts. Workgroups, domain joining, browsing, hostname update and other features of the SMB suite will be restricted to your network due those limitations. It shall work in a local network environment but not over TCP/IP. However, to overcome this issue, NBT (NetBIOS over TCP/IP) and WINS servers where implemented so, things like hostname updates could be done on larger networks where routing is needed.
SMB itself is just an upper-layer protocol (presentation & application), and it will consume lower-layer protocol (network, transport, session) services. It will work across networks, but it heavily depends on the implementation/version of SMB you are using, and the operating system.
tl,dr; It's better to use other protocols like WebDAV, sftp, scp or ftp.