I’m not a fan of pre-preparing. My honest advice would be just relax, enjoy your summer and tackle third year as it comes.
Third year is quite different to first and second and you really need to begin it before you can get a feel of how to revise and how you learn. Clinical medicine does need to be learnt surrounding the clinical environment so there won’t be a lot you’ll be able to do to help yourself in advance. It is a lot busier than 1st and 2nd so you’ll be glad you made the most of this free time to relax while you still have the chance.
I can’t think of anything specific that would have helped me if I had done before beginning third year, apart from learning your physical examinations from year 1 and 2 inside out. Make sure you learn all the signs you’re looking for with each part of the examination and even practise it with family and friends if possible.
Looking over 1st and 2nd year content won’t help as in third year you re-cover the conditions but from a more clinical perspective.