Sorry for the late response, we had to do some testing before giving you good news.
The Bluetooth Plugin now also works with smartphones, tablets, etc. to stream music from. The input from the bluetooth device is received by the Pulse Audioserver and streamed to the next audio device the server grabs (usually the 3.5mm jack).
If you use a sound card, you still need to select the Pulse Audioserver as the output device in Squeezelite, but you also have to „deactivate the integrated audio on Pi3“ in the Raspberry Setttings to make sure that the audio jack is not used as the output device.
Also, if you are using a screen, make sure to remove the tick on „Autostart Desktop“ under Settings/Reboot, otherwise two conflicting instances of the X Server might be started.