You've got two speakers in the bedroom and one DVC speaker in the bathroom.

So clearly you want to hook this up so you make use of the two outputs from the Sonos player and end up with stereo in the bedroom and still have both channels in the bathroom but since they are coming from one speaker it will be mono.

You should have one pair of wires running from one output of the Sonos (say Left) to one of the speakers in the bedroom and then on to speaker in the bathroom connecting to one voice coil.

You should have the other pair of wires running from the other Sonos output to the other speaker in the bedroom and then on to the speaker in the bathroom connecting to the other voice coil.

If they are all 8 ohm speakers you should have no "red-to-black" connections anywhere.

Again:

One bedroom speaker in parallel with one voice coil of the bathroom speaker to one Sonos output.

The other bedroom speaker in parallel with the other voice coil of the bathroom speaker to the other Sonos output.