The P3 was retired some time ago, so I'm not sure why the OP is asking about it unless he already owns it or can buy it used. It was retired about 5 years ago. Some of the specs are not very good, crosstalk of 55 db at 20 kHz, S/N ratio of 92 db A weighted. Apparently it didn't sell well thus the retirement.
The P5 has crossover options for the sub and main speakers that greatly improve the sound quality in any room. The volume control is implemented very well too, allowing for very small increases and allowing for much more of the dial range to be employed.
I mention these features because they do affect the sound and also because I recently took delivery of the P5. I can say I have also owned the P7, and I prefer the P5. There was something about the P7 for me in my system that was just off putting. The P5 offers excellent detail, giving a deeper look into the music and imaging with air that adds to the realism.
I know on paper the P7 should be the better unit, but I have to disagree with the suggestion to opt for it over the P5. As for the room inside the P5, it was designed from the start to drop a 160 wpc amp section in, albeit in a taller cabinet.
And IMHO, the P7 is not worth double the price of the P5 if one simply wants 2 channel.
