VM software and the Virtual disks created in them offer nearly 100% OS functionality as if it was installed directly on the physical drive**. I use VMWare workstation on windows and VMWare Fusion on MacOS (there’s also a version for Linux) which works great, and if you don’t want to pay for a VM software, Canonical (creators of Ubuntu Distro) makes VirtualBox which is free.

I hope that helps and good I have to concur with LonerT here that especially with newer macs but also applying to older models, unless you are just diehard about using all 3 OS’s on the same HDD/SSD (which there’s very few reasons other than preference that you would need to)** dual booting only MacOS and Windows on the same drive and using Linux via VMware or VirtualBox would be the most efficient and far less worry-some alternative to having 3 OS’s of different filesystem types (MacOS being a fork to UNIX but has a proprietary FS type, Windows always being NTFS, and Linux being EXT3 and EXT4) and the problem of maintaining functional boot sequences and not overwriting or breaking one of the OS’s Boot Partitions (within their respective OS). I ended up removing my Ubuntu partition later on, and kept my Windows 7 Pro, and just got VMware Fusion on my MacOS partition and installed some Linux Distros from there and works great (which could be an option if you can flex with that compromise) but either way looks like you may be having to delete both the Ubuntu and Windows partitions and start over. Although this guide is from 2017, I am fairly sure it’s highly relevant as I doubt the parts of MacOS/Windows 10/Ubuntu have changed from how this lays out so you get a 100% working triple boot setup. Yea I have a mid-2010 MacBook Pro i7 15” running MacOS Sierra 10.12, and did a triple boot for a while using Windows 7 Pro, and Ubuntu 16 (or 17?) LTS at the time, and while your boot order looks correct not sure if you did it the right way during the ubuntu install, as its super imperative to do this to the T so as not to cause conflict between Windows and Linux distros and their boot managers.

Triple boot mac windows linux