Install macOS, Linux, Windows in the same USB External SSD
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.