Create RAID from a running system

Hi,

I have an IBM server where i've recently installed CentOS 6.9.
On this server i've installed a Postfix mail server which serves as an archive for our main Qmail server. The archiving is done by imap-sync (which is a great tool btw).

The problem i have now is that when i have installed the system i had only one 2 tb hdd available and i needed to setup the server because our qmail server run out of free space. Obviously i can't rely on this single hdd so i want to setup a linux software RAID 1 or other RAID setup (i haven't got much knowledge about RAID systems) made of 2 new hdd plus the one in which CentOS is now installed.
How can i setup the RAID between the three disks without losing any data?

Thanks in advance.