3CX is no longer creating client-side integrations, and have depreciated those integrations that they created prior to releasing the server-side integration.
Frankly, I'm not sure how long 3CX will continue to include client-side plugin technology in 3CX Phone for Windows. However, building a custom plugin for 3CX Phone for Windows and HubSpot is an option. Alternatively, if you are concerned about the longevity of this feature, you could build your own application that subscribes to call events (using the 3CX Call Control API), and have that application perform the popup. This approach would work with any client (web, windows or physical phone).
Client-side applications always have one major drawback -- installing and maintaining the client application on every PC. Further, you have to consider that 3CX and HubSpot will make breaking changes from time-to-time so this is not a write-once-and -done situation. You will have to make code changes occasionally.
We currently use HubSpot internally here at VoIPTools and have written a custom integration. However, depending on the features of the newly announced 3CX help desk feature, we might start using that rather than HubSpot if it meets our needs.