Is it possible to use a potentiometer as a DP3T style control

Normally reactive components such as inductors and capacitors are used to provide attenuation within/outside of specific frequency ranges and this by definition is how cross-overs are built.

Reactive components are favorable because they do not dissipate power apart from minimal R losses.

If you want to attenuate the signal being driven into a power load such as a speaker then I believe you will need a fairly robust potentiometer. If you are delivering 40 watts into a speaker, and you want to attenuate it by -3dB, then your potentiometer will need to be 8ohms and capable of handling 20 watts. Something the size of a high school lab rheostat would be suitable, but impractical and slightly inductive. I would suggest you consider attenuation of the signal entering your amplifier rather than the signal coming out of it.

If however you're dealing with 250mW headphone speakers then the potentiometer will work, however you may want to consider a Logarithmic type as the parallel impedance equation is not linear with differences in R.

Consider this: -

Sound level is measured in dB and, a 10 dB increase/decrease in signal equates to a doubling/halving of loudness as perceived by the ear/brain.

Look at the picture above and ask yourself which is the better choice for smooth (coupled with extensive) volume controller. Below are the Fletcher Munson curves showing the full range of decibels that a human can comfortably hear. Note, that unless your stereo system is very powerful, a range of 100 dB is "about right" for volume control. The Fletcher Munson curves also relate loudness to the pitch of a sound. Note also that the curves are all normalized to 1kHz in 10 db steps: -

Approximately every 10% of travel of the wiper on the LOG potentiometer can reduce/increase the volume by 10 dB whereas a LIN pot will need to move all the way down to its middle position before it's reduced the volume by only 6 dB! When a linear pot is near the bottom end of its travel (sub 1% of movement left) it will be making massive jumps in dB attenuation for just a tiny movement hence it would become very difficult to set the volume accurately at a low level.

It's also worth pointing out that a LOG pot is only able to cope with so much dynamic range of adjustment before it does the same (below -100 dB) but, the point is, this will hardly be noticeable at the tiny, quiet end of its travel.

You might also note that the markings on a pot such as CW and CCW tell you which end of a pot is the ground end and the high-volume end. CW = clock wise and CCW is counter clock wise end points for the wiper.