Scratch

Obviously the subject of this post is a bit tongue in cheek but I do need some help trying to recreate a famous puzzle from G.E.B. in a Scratch project. 

Here is my problem.  I have a variable that consists of a series of letters.  The number of letters this variable contains varys.  It might be only 1 letter but it might be 30 letters long. 

If the same 3 letters appear in a row within the variable, I want to be able replace these letters with 1 letter, but at the same location that the 3 letters originally appeared. 

I have figured out how to write a script that would have Scratch search the variable letter by letter determining if the same 3 letters appear in a row. 

What I can't figure out is how to replace these 3 letters with a 1 different letter but maintain the position of the remaining letters within the variable. 

Any ideas?

For those that are curious, I am trying to recreate the MU puzzle from Godel Escher Bach by Douglas Hofstadter.  I have Rule 1 and Rule 2 working perfectly, and the above question is to finish the script for Rule 3.  Obviously, the same script for Rule 3 will work for Rule 4 with a little tweaking. 

Thanks.