The signal light bulbs are switched by the FEM (in the front) and the REM (in the back), but AFAIK they are controlled by the cluster. It is my understanding that the micro in the cluster reads the turn signal switches and tells the FEM and the REM when to turn the bulbs on and off.
If we has access to the source code for the micro in the cluster and the tools to compile it and program it into the cluster micro, then it would be really easy to do. Since we don't and never will, it gets more complicated. You would have to add a device (/w microprocessor) between the turn signal switches and the cluster that would prolong the signal enough for 3 blinks. Doable, but not worth the effort in my opinion.
As for the out-of-dateness of it, the design of the LS was started in 1998. I don't think anyone had the 3-blink thing then. The refresh was started in 2002 (maybe 2001?), and this didn't make the feature list then. By or before 2004, they knew the LS would be discontinued very soon.