Here is my 3 tall Aguilar GS112 stack.
I'm using a sansamp BDDI into a yamaha power amp 400 watts per side 4 ohms.
The reason I'm using 3 is not neccesarily for volume, although it is nice to have when needed, but to get one of the cabs up to closer ear level without losing coupling with the stage. The 3 cabs together are about as high as an Ampeg 610hlf cab. I am running the top 2 cabs (GS112 and GS112NT) on one side of the amp 200 watts each) , and the bottom cab (GS112nt) on the other side of the power amp at 275 watts 8ohm.
On boomy stages I am able to turn down the bottom cab reducing volume and low end.
Also comparing apples to apples, an Aguilar GS412 is
$1100 street price. I bought my GS112 for $400, one GS112NT for $350 and one GS112Nt (blem, still haven't found the flaw) for $300, free shipping on all cabs btw. Grand total
$1050 I was also able to buy each cab individually as I could afford them instead of plunking down the whole wad at once.
I would rather carry around three 40 pound cabs that I can easily lift over my head or onto a 4 foot high stage Than a 120 pound 412 cab. I also have the option of only as many cabs as I see fit. Can't do that with the 412, you're stuck with the big cab. If I have a long distance to haul the cabs after I unload from my truck, I have a hand cart that does the job nicely. Going up and down stairs? No contest.
So since the original poster already has two 112 cabs I still think he would be better off just getting one or two more and enjoy the flexability. All of this of course, is just my opinion.