The Pi 3 can be a great choice for a backup (even an offsite backup) but not as a primary server. USB 2.0 and RAID simply do not mix, as I learned through a lot of headaches.
The Pi 4 is a far better candidate for RAID due to the USB 3.0 connections.
But, even then, it's not a particularly elegant solution because plugging in a bunch of hard drives via USB is an unwieldy mess. You can do it, but it's not pretty. I'd love to see some proper enclosures for a Raspberry Pi NAS.