Welcome to the Ranch.
You've made a common mistake with arrays: you have declared the array (line 10 of your second code snippet), but you haven't put any Cat objects in it. You have an array with three null elements.
You'll need to create new Cat objects and put them in the array, before you try to do something like
MyCat1[i].isDeclawed() - if MyCat1[i] is null in that line, you'll get a NullPointerException.
A NullPointerException usually happens because you try to call a method on a variable that is null.
In the first for-loop (lines 13-27)
you should, after getting values from the user, create a Cat object with those values and put them in the array at position i.