You should be possible to install, macOS Catalina 320GB, Windows 10 320GB, Linux CentOS 7 320GB in the same external SSD. You probably will have to use the CLI to install Windows 10 to the external SSD.
Below is a suggested GPT partitioning for the drive.
EFI
for macOS Catalina and CentOS 7
Apple_APFS
for macOS Catalina
EFI
for Windows 10
Microsoft Basic Data
for Windows 10
Microsoft Basic Data
for CentOS 7
Linux LVM
for CentOS 7
I would suggest creating partitions 1 through 4 before installing any operating systems. Leave free space where partitions 5 and 6 will eventually reside. Next, install macOS followed by Windows 10. Finally, install CentOS. Here, I would let CentOS create its own partitions (5 and 6) in the free space.
I do not know what your experience is with respect to installing operating systems. I can expand on this answer if necessary.