[madbrain]

Is there any chance one could use Kodi on the Pi3 to stream the audio to the Chromecast audio instead of HDMI, while still streaming the video over HDMI ?

Not likely. Kodi is all open-source, and Chromecast is proprietary/closed software from Google.
Those two types do not mix will in ourview. Thus, Kodi has no support for Chromecast, for audio or video.

Klojum - didn't see a followup to my last response.

I see that Kodi already supports UPnP/DLNA players. There is even software I can run (on a Pi, or other device) that will create a DLNA server and pipe audio to my Chromecast audio.

My use case is one where the audio device and video device aren't one and the same.
In my case, I want the video to go for HDMI, and the audio over to the DLNA player.
It's possible others might have the opposite need of mine, where the local device running Kodi sends the audio, but the video stream needs to be sent to a remote device.
I understand this may not be a very common use case, but for those with multi-room audio / in-wall/ceiling speakers, it is likely not that uncommon. I have over 50 speakers in my house, but there isn't an amplifier for speakers in every room. Only 2 rooms have amps that feed most of the speakers. All those amps have audio streamers attached - in my case, CCAs, but they really could be Rpi with DLNA servers instead, or other open-devices.