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in have 3 subs compared to having 2......any differences in spl or sound quality or is it good just for looks

 
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every sub adds 3db of sound
every time you double the power it adds 3 db

 
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to those of you not knowing the standard of 3 db's, its a doubling of the pressure... you wont hear it twice as loud, but the pressure will be twice as high, but thats saying that you have 2 subs, each being pushed by 300w, and add another sub, and put another 300 w to it... if you just split your existing power the increase in spl wont be as high, but you may see less distortion due to the fact that you'll have to push it less to get to the same level of bass..

 
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so if your running 30 10" with 30,000 watts of power hitting 170 dB, and add one more 10" and another killowatt, youll meter at 173.

in terms of output, every time you double the number of woofers in your system (1 to 2, 2 to 4, 3 to 6, 4 to 8) AND double yoru power, you double your output. on an SPL meter this will translate to a 6 dB increase, assuming 100% efficiency.

the difference between 2 and three woofers all depends on box design, and the power you have available. I personally wouldnt consider getting an additional woofer an upgrade if you are currently running two. if you are just planning at this point, then getting a trio of woofers may be worth considering, but just adding a third if you already have two would mean rebuilding the box and maybe purchasing a new amp, which may or may not be in your best intrests.

certainly not JUST for looks, but like everything, it has a piece of it too. do a search, and youll find one of mohawk and my threads about output and energy, and the benifits and gains from various power and subwoofer combinations.