Oh, it should be substantially brighter than the KPR112.
I don't recall a rock solid answer as to the number of lumens that KPR112 or 113 put out when overdriven with 9V, but my highly calibrated, super scientific WAG is that it's just over 120 lumens.
The GH24 is rated to put out 200 lumens when driven to 6V (only 2 123s) so I would guess at least 300 lumens on 3x123. I've never used one but several folks have stated that the GH24 is quite yellow when driven at spec. Keep in mind, this bulb run on 2x123s shouldn't melt the reflector or lens but on 3 123s, heat is going to be a problem. This is about the time we start looking at aluiminum reflectors and some sort of replacement lens such as the UCL from flashlightlens.com
Mike2004, I've built two of the 3x123 + KPR112s. I've never dropped either of them, but the first one I built is still on the 112 bulb I put in the day after I built it. Originally used a 113 bulb. I'm on the third set of batteries. And all three sets were 100% fresh SureFires. Again, I personally wouldn't rely on the 3x123 with 112 or 113 bulb as my only flashlight, but is far from horribly unreliable. For a similar light form factor, with 3x123 driving a KPR118, they bulb should last over 25 hours or as I suggested above, have the brighter and more overdriven 112 or 113 bulb actually installed and keep a 118 bulb in the tailcap in case the 112/113 blows.
Also, 3x123 is only 9 volts, not 12. You can also fit 4x123 in the 2C driving a KPR118 but I haven't tried that.