Triple boot Catalina 10.15.7, windows 10 (not bootcamp but NTFS), Ubuntu 20.10
Installing Ubuntu 20.10 when macOS 10.15.7 Catalina and Windows 10 (version 20H2) have already been installed.
This answer assumes Windows is installed to BIOS boot. This can be verified by viewing the contents of the Windows Disk Management application window. Below is an example when Windows BIOS boots. Volume (Disk 0 partition 1) has a status of Healthy (GPT Protective Partition).


Note: To get a better look at an image, either click on the image or open the image in a new window.

Below is what would appear when Windows EFI boots. Volume (Disk 0 partition 1) has a status of Healthy (EFI System Partition).

Below are the basic steps.

Use the macOS Disk Utility application or diskutil command to create a ExFAT partition which will be to contain the Ubuntu installation. Below is an example for the use of the diskutil command.
The output from diskutil list disk0 is shown below. This shows the partitioning before adding the new ExFAT partition.
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.2 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         578.6 GB   disk0s2
   3:       Microsoft Basic Data OS                      578.6 GB   disk0s3

The command given below gives approximately have the space used by the APFS container to the new ExFAT partition. You may want to replace 289G with your desired new size for disk0s2 container partition occupied by macOS Catalina.
diskutil apfs resizecontainer disk0s2 289G exfat DUMMY 0 

The output from diskutil list disk0 is shown below. This shows the partitioning after adding the new ExFAT partition.
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.2 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         289.0 GB   disk0s2
   3:       Microsoft Basic Data DUMMY                   289.6 GB   disk0s5
   4:       Microsoft Basic Data OS                      578.6 GB   disk0s3


Install Ubuntu. Below are some pointers.
When you reach the window shown below, use the following settings.

When you reach the window shown below, use the following settings.

When you reach the window shown below, highlight the /dev/sda3 device as shown below and then click on the Change... button.

In the Edit partition popup, use the following setting. When finished, click on the OK button.

Click on the Install Now button shown in the image below.

In the Write the changes to disks? popup shown below, click on the Continue button.


Continue and finish installing Ubuntu.

The installation of Ubuntu has changed the partition tables resulting in an unbootable Windows. This step will provide the commands needed to fix this problem. While booted to Ubuntu, press the control+option+T key combination to open a Terminal application window. Enter the following command.
sudo gdisk /dev/sda

The gdisk command is interactive. Below are the commands to enter.
t
3
8300
r
h
2 3 4
y
ff
n
83
n
07
y
w
y

When finished enter the command given below to close the Terminal application window.
exit