Between the top 3 and the last 2 positions, I want Insert ~- in front of each character. how to?

You have posted few scripts already - don't understand why you should have a problem with this? 

You need to get a reference to your word - then use insertionPoints collection - and count from the back / end - or from the start, but from the back would be easier:

myWord.insertionPoints[-3].contents = discretionary_hyphen_code;

You need to remember, that "-1" is the insertion point AFTER the last character "-2" is BEFORE last character and so on. 

You might want to do it in the loop.  

Collection of insertionPoints always has one more element than collection of characters - in ANY text object.

Or it might be also done with GREP.