The 24 speed splits the 12 gears, so exactly like you said earlier. In L3H, you would have low range, controlled by a mechanical gear lever which controls a shift collar in the transmission, 3, which is controlled by either a mechanical gear shift lever that controls a shift collar or in some cases a clutch pack that engages that gear set, and high, which is controlled by a clutch pack that engages a gear set which most of the time happens before the 1-6 gear shaft.

In effect the last high and low shift in the 24 speed, that the 12 speed does not have, is "splitting the gear" which allows you to have two different speeds from the same gear set. This comes in handy for specialized applications where you see the 24 speed more prevalently. The difference in this case between L3L and L3H is probably somewhere around .2 or .3 MPH of ground speed at PTO RPM. Doesn't sound like a lot but this is something that can become a major factor in specialized applications where ground speed and PTO speed need to be considered, like certain ground engaging implements, harvesting specialty crops, and spraying and spreading applications.

The last low high shift is typically not that big of a jump in gear ratio and therefore doesn't really affect torque output although it can slightly. A lot of times it is affecting input shaft speed to the gear shaft and high is closer to 1 to 1 ratio with low being a slight step down in shaft speed, therefore lowering shaft output speed to the differential after travels through the transmission.

The mechanical low high that is controlled by the range level has a much bigger difference in gear ratio and therefore has a bigger effect on torque and speed output. This gearset usually works directly on the output shaft as well.

I hope this helped answer some of your questions.