Section L is now at Issue 14, the main changes being the removal of initial issue of aircraft licences above 5700 kgs, due to JAR-66, and the reworking of the exam and experience requirements to better alig with the proposals for light aircraft in JAR-66 NPA1.
The three years is a mix of several things. Three years of general aviation engineering experience of which 12 months must be specific to the basic licence ratings that you are going to sit. Quite right that you could look over someone's shoulders but where is the fun with that on a cold dark night.
The main criteria is obviously to become familiar with the practical, theoretical and diagnostic principles of the systems you want to certify for. Maybe I'm just an old fashioned enginer and I have got it wrong in trying to do the best I can. Whatever hapened to self-study?