3 subs

https://www.the12volt.com/caraudio/boxcalcs.asp

This is a handy calculator for figuring impedance total on any combination of subs.  Three SVC 4-ohm subs would connect in parallel to 1.33 ohms.  Okay to use with a mono amp rated to 1 ohm.

Figure that you have to wire each sub to either 2 ohm parallel or 8 ohm series.  Then just plug either 2 or 8 into the calculator instead of 4.

With the two subs you have now, you probably are running them parallel/parallel to 1 ohm.  Adding the third sub will give you 2.67 ohms with series/parallel.....a lot easier on the amp.  The added sub (cone area) will give you a couple db but the higher load on the amp will make it produce less power.  You'll end up with about the same db as you had before but the amp will run cooler and with better specs.  Any mono amp will operate fine into a 2.67 ohm load.

Yes they are 1 ohm stable... The older BP series (BP-600.1, 1200.1, 300.1 and 150.1) are all 1 ohm stable, but will output the same amount of power at 1 ohm as it would at 2 ohms.

Features

• High-Efficiency Switching Power Supply
• "Direct Connect" Terminal Block
• 2 or 4 Channel Inputs Allow for Non-Fading Bass
• Full-Size Die-Cast Aluminum Chassis
• Variable Low-Pass Crossover
• Variable Bass Boost
• 2-Ohm Optimized, 1-Ohm Stable

That's from sounddomain's site: http://www.cardomain.com/item/JBLBP1501Z

You won't need a 1 ohm stable amp though because you can wire all three of them for 2.66 ohms like DYohn said.