Okay, first things first, you've got your terms mixed up (its okay, it happens, its easy to do with all the freakin acronyms). GRUB is not an OS. GRUB is a bootloader, a tiny tiny tiny piece of software that runs when you first boot the machine (right after the BIOS screen) and allows you to select what OS you wanna boot. (Basically the screen that you press "Debian GNU/Linux", yeah thats GRUB there)
Debian is the operating system; a distribution of GNU/Linux.
Now that that's out of the way, here's what I suggest.
Since you want to run your servers in virtual machines, the easiest way to do that is Proxmox VE. It allows you to run Linux (including Debian, CentOS, Ubuntu, RHEL, Fedora, yeah pretty much every distro) in containers instead of traditional VMs, saving overhead and hassle. You can also run Windows/BSD/other OS VMs inside full virtual machines, using a hypervisor called KVM. KVM is pretty much like Virtualbox.
The reason I'd use Proxmox is because of how easy it is for new Linux users (its what I started with and I swear it grows with you better than any software I've ever used, I'm still running it on an FX-6300 machine to this day). It's pretty much VirtualBox in your web browser, running on the server. You can manage all your VMs from there, make new ones, back them up, change their resources (RAM, CPU, etc) and just about anything else, all from a convenient little web gui.
If I were you, I'd get Proxmox installed and set up your first VM by watching Wendell's video he did about it. It's really an excellent piece of software, and he does a great job of explaining how it all works and how to get the most use out of it.
Here's a link to Wendell's video. Click me I'm a link
Okay, now the only thing you may have issues with from using Proxmox is RAID. If you can do what Wendell said and grab a cheap LSI RAID controller off eBay, that'd probably work best.
But if you can't, here's what I'd do: note that for this I assume you want to run the OS off the SSD and store your VMs and files on the 3TB drives. If thats not right correct me, but that seemed logical enough
Okay, so when you install Proxmox, remove (like, physically unplug from your mobo) all the drives except the SSD. Install it to your SSD. Then boot it up and verify that you can follow Wendell's tutorial on creating a simple Debian VM.
If that works, and you can access the machine's web GUI via the network, you can then use PuTTY (SSH, if you're new to this) to get into the Proxmox host machine. It's just a modified Debian install, so you can configure your software RAID via the command line there, just like on a normal Debian install. I've never used RAID with Proxmox so I can't confirm that for sure, but as long as you're not storing any important stuff on the drives already, worst case you reinstall the OS if something breaks.
Once you get the RAID configured, I'd mount it as something like /raidarray, and then go to Proxmox's webGUI and mount that as "raidarray" under Storage. Then you can store your VMs on the RAID array (it lets you choose the storage location for 'em when you make them) and your host OS will run on the SSD.
TL;DR - Please read the whole post but the summary is pretty much "Use Proxmox, install it on your SSD, then configure the software RAID from the command line and you'll be all set. From there, make a VM for each thing you wanna do."
Let me know if you run into any issues. I've never done RAID before on Proxmox so I can't promise this will work, but just don't have anything life-alteringly important on those disks until you're sure its ready, and you'll be fine. Worst case reinstall the OS. Hopefully it works out for you!
Maybe someone who's a bit more knowledgeable on this can confirm this works?