If the disks have already been used (ex. installing over an existing OS) then choose "reformat" on each boot disk. This will not actually erasre any data until the process commited at the end.
Select each disk to be used as a boot device
Create a 1GB partition for the first raid device on each disk. Leave the partition "unformatted"
Create a 4GB partition for the second raid device on each disk. Leave the partition "unformatted"
Create a partition that fills the rest of the space for the third raid device on each disk. Leave the partition "unformatted"
Select "Create Software Raid (md)" to create first raid device, Choose 1GB partition on each disk and name the device "boot"
Select "Create Software Raid (md)" to create second raid device, Choose 4GB partition on each disk and name the device "swap"
Select "Create Software Raid (md)" to create third raid device, Choose teh last partition on each disk and name the device "root"
Verify all three boot devices, boot,swap and root are created
Create partition on boot raid device. Leave "size" blank, select "ext4" and change mount point to "/boot"
Create partition on swap raid device. Leave "size" blank, format as "swap".
Create partition on root raid device. Leave "size" blank, format as "ext4" and change mount point to "/"
Verify that each raid device is created and all three are formatted, the "Done" button will activate at the bottom of the screen
Confirm Storage Layout, WARNING, there is not turning back as existing data WILL be overwritten.
Finish Installation
Create local user
Check box to install OpenSSH, leave user import blank
Do not install any snap packages as we will be removing snapd completely after installation
Installation will begin. Select "View Full Log" to see more detailed process logging. Select "Reboot Now" when installation finishes
Once the installation is complete, remove the ISO USB or the Virtual Mount and reboot the system.
Once the installation is complete you should be able to boot into the Ubuntu 20.04 OS.
Boot into system and verify raid devices
Troubleshooting
If installer is stuck trying to "Remove Media" It is safe to power reset the server. Either detach the ISO through the IPMI or physically remove USB stick from server.
If the installed fails it may mean you will need to boot the OS into a Live ISO to wipe the boot drives before continuing with the installation.