I just ordered this laptop waiting for it to arrive but right now my old HP Envy, I did the following:
- Create bootable USB of Mac High Sierra per the guide on page 1
- Partition the hard drive into 2 partitions using Disk Utility
- Format one as Windows FAT and the other as Mac OS X Journaled (it's important to use Disk Utility on Mac to do this as the EFI partition has to be at least 200MB for certain Disk Utility to function)
- Install Mac onto the second partition
- When computer reboots, boot using USB into Clover so you can boot onto new Mac partition on the Hard Drive
- Install Clover onto the new partition
- Verify Clover boots without USB
- Boot Windows using created USB media and wipe the Windows FAT partition and allow Windows setup to create the necessary partitions for UEFI GPT
- When Windows completes, you should be able to further partition the C: drive to make room for Ubuntu
- Boot using Ubuntu live cd and you should be able to use the remaining unallocated space to install
- During Ubuntu install, I kept the original Grub location and was able to detect all 3 partitions in Clover to boot from whatever one I want.
Since I don't have this laptop quite yet, I can't guarantee it will work exactly the same way, especially if the config.plist file hides Windows or Ubuntu from being displayed in the boot process, but hope this helps.
Edit: Here is my partition scheme with all 3 OS's installed. I am currently using a 512GB EVO 850 SSD which will replace the 1TB my Dell will have but at least I will have 128GB of extra storage with the SSD that's included.