The first one is wrong (more than one court). The other two are correct, but I think the second is less clumsy. Better would be: This paper compares the case law of the international and national courts.

Tricky question. If the thing being possessed (the case law) is the same for each court, then you will use the first option. However, if they are different for each court, you will use the third option. In this case I would recommend the third option "This paper compares the international court's and the national court's case law" This post may help to explain the rule a little better: https://www.quickanddirtytips.com/education/grammar/compound-possession?page=1 Another option to consider is to change the structure of the sentence to make it more clear. Maybe something like: This paper compares the case law between the national court and the international court. P.S. Yes, you should use 'among' because there are more than two options. I hope this is helpful! James