I'm considering a few used EFI ES-3000 spectros on the market.

From my limited research, it seems that the EFI line is a basically an i1Pro 3, just with different branding. I see that Ted confirmed this back in 2023.
https://www.avsforum.com/threads/efi-es-3000-vs-i1-pro-3.3289757/

Can anyone confirm whether an EFI spectro will connect seamlessly with ColourSpace's i1Pro 3 probe selection? I'm curious if there is any internal software that would block this communication with Light Illusion.

Yes, works exactly the same as the xrite branded version.
It has no 'lock keys', as with i1D3 probes.

Steve

Excellent, thanks Steve! That model just arrived in the mail today and I can confirm that it works. Thanks so much.