its all relative man.
a 15" cone will move 15" of air, two 12's = 24" of cone surface.
However it really has nothing to do with yuor question.
3 10's can sound better then a 15", or the 15" can sound better then the 3 10's.
Its about MATCHING.
Match the amp power to the sub RMS, then research and decide what type of enclosure is right for you. I constantly see cheaper subs with the right amount of power to them in a quality enclosure outperform high $$$ equipment installed incorrectly.
I use a cheapo MTX 12" 2ohm sub in a 1.25 cu ft sealed enclosure. the Sub asks for $250W RMS, so I used a 300 watt sub amp at 2 ohm to power it. Sounds amazing, takes up little space, and fits my needs perfectly. I prefer sealed enclosures these days for a punchier bass that is more true to life to the music, my days of booming bass are gone, I am older now and wiser
Besides I need my cargo space.
in my boat I have a series I rockford 12" (cheapo sub incase it gets wet) with a single MTX amp in a LARGE, TUNED, PORTED enclosure. Why? Boat has no roof or cabin, so the more low end extnesion I can get the better. It has a TON of bass, way more then my home or car and it sounds GREAT.
Over the years I have built probably 60-70 enclosures, they get better each time I build one. If you want your sub to sound RIGHT, no matter the size or number of subs, it is all about sending it the correct amount of power and building the PROPER enclosure for your purpose and driver.
the factory recommended box dimensions ALWAYS work well.
Research is the key. Pick the size of the subs you want based on how they fit in the vehicle, the amp you will power them with, etc.