No-one expects you to fill in all the chapters.
Your training was likely to have been biased to either mech or Avionic trades and your experiences should be recorded accordingly.
But you should be able to clearly demonstrate how much work you have done in those three years against the majority of chapters for your given trade.
For a basic AML the CAA willl be looking for a wide range of work within each appropriate chapter. So repeating the same thing many times may fill a page but not show a variety of experience.
However you can be smart about how you log the jobs you have done. You may be able to cross feed some jobs that involve different system tests, such as: an Air Conditioning job may lead to electrical sensor connections and engine runs - all from the same task. You still have to do the Air Con job, but where else did you go during that job?
Think about what you did and log it properly.
Additional stuff:
As a trainee (of however many years) you may be very lucky to do a lot of line work/training - especially if your tied to a fixed base. The CAA generally know what type of work you're doing and what type of work you can get from the organisation that is sponsoring you. Be patient, keep logging what you do. If you are short of anything your QM or training supervisor should sort you out. Show them your book and ask for advice on what you need.
Best of luck mate.