Triple boot macOS High Sierra, Ubuntu, and Windows without rEFInd
Alternative Second Answer

This second answer differs from my first answer in the following ways.


The second EFI partition is replaced by a HFS+ partition.
The label displayed in the Startup Manager will be "Ubuntu" instead of the generic "EFI Boot". 


I will assume the following:


You have install Ubuntu to use the EFI boot method.
You are using grub to boot Ubuntu
Grub boots from the EFI partition.
There is only one EFI partition.
The EFI partition is the first partition on the Disk.
There is only one disk involved and the disk is internal.
rEFInd is installed in the EFI partition.


I will offer two ways to solve your problem. Each has it pros and cons.

The Mac Startup Manager will display an operating system if you place the .efi file in the right location. This location is the System/Library/CoreServices folder of the volume in a HFS+ partition. The file name must be boot.efi. Furthermore, you can customize the icon displayed for the operating system by placing a .VolumeIcon.icns file in the root folder of the volume in the HFS+ partition. 

The First Way.


  Note: Here, rEFInd is still being used, but not as a replacement for the Startup Manager. You will be able to use a firmware password. In fact, no rEFInd menus are displayed. The rEFInd boot manager just silently boots grub (which boots Ubuntu).



Download rEFInd from this SourceForge website. I assume the downloaded file is named refind-bin-0.11.2.zip and this file was downloaded to your ~/Downloads folder.
Create a new HFS+ partition. Instructions are given in the section "Adding a HFS+ Partition".
Mount the new HFS+ partition, by entering the following command.

sudo  diskutil  mount  disk0s3


The new HFS+ partition will have the volume name Ubuntu.
Install rEFInd into the volume on the new HFS+ partition. 

cd  ~/Downloads/refind-bin-0.11.2
./refind-install  --ownhfs  /dev/disk0s3



  Note: The error message Could not set boot device property: 0xe00002bc was expected and can be ignored.

Configure the TextEdit application. Open TextEdit, then navigate to the "Preferences..." window. Uncheck all the "Options", as shown below. When finished, quit TextEdit.


Use the command below to navigate to the folder containing the refind.conf file. 

cd  /Volumes/Ubuntu/System/Library/CoreServices


Make a backup copy of this file.

sudo  cp  refind.conf  refind.conf.orignal


Add write permissions to the CoreServices folder and the refind.conf file.

sudo  chmod  a+w  .  refind.conf


Open the file in the TextEdit application.

open  -e  refind.conf

In this volume, edit the System/Library/CoreServices/refind.conf file to silently boot grub (which will intern boot Ubuntu). This can be accomplished by adding the following lines to the end of the refind.conf file. You should be able to just cut and paste these lines. When finished, save the changes, then quit TextEdit.

#
# Added to make rEFInd silent. 
#
menuentry "Ubuntu" {
    icon \System\Library\CoreServices\icons\os_ubuntu.png
    volume "EFI"
    loader \EFI\ubuntu\grubx64.efi
    ostype Linux
    graphics on
}
timeout -1
hideui all
scanfor manual

Unmount the new volume named Ubuntu, by entering the following commands.

cd  ~
diskutil  unmount  disk0s3



Pros: Ubuntu is unaffected, so any Ubuntu updates will not require attention. 

Cons: You need to use the third party boot manager rEFInd.

The Second Way.


Create a new HFS+ partition. Instructions are given in the section "Adding a HFS+ Partition".
Mount the original EFI partition and the new HFS+ partition, by entering the following commands.

sudo  diskutil  mount  disk0s1
sudo  diskutil  mount  disk0s3


The original EFI partition will have the name EFI and the new HFS+ partition will have the name Ubuntu.
In the new volume named Ubuntu, create the folders EFI/ubuntu and System/Library/CoreServices, by entering the following commands.

mkdir  -p  /Volumes/Ubuntu/EFI/ubuntu
mkdir  -p  /Volumes/Ubuntu/System/Library/CoreServices

Copy the files grub.cfg and grubx64.efi to the new volume named Ubuntu. The commands needed are given below.

cp  /Volumes/EFI/EFI/ubuntu/grub.cfg  /Volumes/Ubuntu/EFI/ubuntu/grub.cfg
cp  /Volumes/EFI/EFI/ubuntu/grubx64.efi  /Volumes/Ubuntu/System/Library/CoreServices/boot.efi

Unmount the original EFI partition and the new HFS+ partition, by entering the following commands.

diskutil  unmount  disk0s1
diskutil  unmount  disk0s3



Pros: You do not need to use the third party boot manager rEFInd.

Cons: If the grubx64.efi or grub.cfg is updated, you have to manually copy these files to new volume named Ubuntu.

Adding a Custom Icon

The steps below will add the following Ubuntu icon to the Startup Menu. 




Download a collection of icons from the sourceforge web site Mac icns. 
Use the Finder application to open the downloaded file mac-icns.dmg. I assume this file downloaded to your ~/Downloads folder.
Mount the volume named Ubuntu by entering the following command.

sudo  diskutil  mount  disk0s3

Use the Finder application to copy the os_ubuntu.icns icon file to the root folder of the volume named Ubuntu. Alternatively, you can use the command shown below.

sudo cp  /Volumes/mac-icns/os_ubuntu.icns  /Volumes/Ubuntu/.

Rename the os_ubuntu.icns icon file to .VolumeIcon.icns. This can be accomplished by entering the following command.

sudo mv  /Volumes/Ubuntu/os_ubuntu.icns  /Volumes/Ubuntu/.VolumeIcon.icns


Note: Files starting with a period (.) do not normally appear in a Finder application window.
Use the Finder application to eject the EFI System partition with the volume name Ubuntu. Alternatively, you can use the command shown below to unmount the volume.

diskutil  unmount  disk0s3

Use the Finder application to eject the volume named mac-icns.


Adding a HFS+ Partition


  Note: Below is based on the output from diskutil list that you provided in your question.



Enter the command below in a Terminal application window. The will create a "Mac OS Extended" volume named "Ubuntu" in a new partition.

sudo  diskutil  apfs  resizeContainer  disk0s2  242300M  HFS+  Ubuntu  200M

Get the volume UUID for the volume named "Ubuntu" by entering the command below.

 diskutil info Ubuntu | grep "Volume UUID"


The output should be similar to what is shown below.

   Volume UUID:              795F14D2-ED1F-3CA0-A004-BD0A27000DC8

Next, prevent this new volume name "Ubuntu" from automatically mounting, by entering the following commands. Substitute your UUID for the one shown below.

sudo  bash
echo  &gt;&gt; /etc/fstab
echo  UUID=795F14D2-ED1F-3CA0-A004-BD0A27000DC8 none hfs rw,noauto &gt;&gt; /etc/fstab
exit



  Note: The correct way, to edit the /etc/fstab file, is to use the command sudo vifs. Using the echo command is just a quick shortcut.

Use the Finder application to eject this new volume. Alternatively, you can enter the command shown below to unmount this volume.

diskutil  unmount  Ubuntu



If you need me to expand on any of the above steps, let me know.