Its normal for them to be quieter with all 3 rather than just 2 (even if its as much as half)
I took an old radio and setup a the same 3-speaker setup for some testing (simply because I couldn't remember exactly how it worked --sound wize-- when I used to have one)
Using a specially modified audio file & playing it to that radio:
1=File with same audio on Left & Right (Normal Mono)
2=File with audio on Right only
3=File with audio on Left only
4=File with audio on Left & Right (but one channel is inverted - what could be called 'Inverted Mono' - and what on an actual surround system would be directed to the back channel by the processor)
*standard 2 speakers setup*
1. Equal Sound from both speakers
2. Sound from Right speaker only
3. Sound from Left speaker only
4. Equal Sound from both speakers
...basically works exactly as you would expect
*the 3 speakers setup*
1. Equal Sound from both speakers & 3rd speaker
2. Normal sound from Right & 3rd speaker, quiet sound from Left
3. Normal sound from Left & 3rd speaker, quiet sound from Right
4. Equal Sound from both speakers & minimal sound from 3rd
...works similar to what I'd expect (and now just as I remember it working from times before).
With a perfectly "clean" input-signal, and a good clean stereo, #4 in this case could have zero sound on the 3rd speaker. (in my case the test stereo is a cheap thing, and the file a LowFi quality one)