Well you need a digital multimeter and follow the instructions here from forum member HKJ: http://www.lygte-info.dk/info/Measurement%20UK.html
This drop-in might work for you but it's not XML. You might not find an XML drop-in that works around 1amp, they tend to be of much higher current. So at low amps getting an XPG or XPE would be better for your 3xAAA light anyway, cheaper too.
Rough runtime: The drop-in below pulls 1200ma on high so if you have 3xAAA in series and Nimh cells of 850mah you can get roughly(very) 42 minutes of runtime. That drop-in is most likely not constant 1200ma, with voltage sag your light will get dimmer and the runtime will increase a bit.
You risk over discharging your NiMh cells with such a low voltage. If you are using alkaline cells then no problem. But NiMh should not be discharged below 0.9v. Or 3AAA x 0.9 = 2.7 volts.
Ok then, now that I know the runtime of an L2i with R5 drop-in on AAA's (thanks ChibiM) the next step is to understand whether the Solarforce or Trustfire R5-A3 is the more powerful torch.
I'm assuming that the Solarforce will win because it has a bigger reflector, and is supplying 3.75v compared to 1.25v from the Trustfire?
I'm a total novice at this so shoot me down if necessary!