From what I've come across, online, these phone are SIP as of firmware load 5.1. How compatible with 3CX they are, is another question. It seems that they require a tftp server to provision...which means probably means that you will have to "construct" your own provisioning template. You may want to start doing some web searches to avoid having to re-invent the wheel. I've found that others have attempted to use the sets with other SIP PBXs.
It might end up being easier than I suspect, but it may not be a road you wish to go down unless you are prepared to spend a lot of time on it.