I suggest you study a few tutorials about arrays and expressions to get a better understanding of your own code.
Also, refer to the official documentation.
construct.net/en/make-games/manuals/construct-3/system-reference/system-expressions
As you can see in the manual, chooseindex expression has nothing to do with arrays:
You can get rid of chooseindex, it's not needed in your case.
chooseindex(Chatposition,Dialuge_System.At(1,1),Dialuge_System.At(1,2),Dialuge_System.At(1,3))
is the same as this:
Dialuge_System.At(1,Chatposition+1)
To make the code easier to read, you can save the result of that expression to a variable, let's call it res. It's up to you if you make it global or local.
Set res to Dialuge_System.At(1,Chatposition+1)
Then you can split the result into sentences using tokenat and tokencount expressions. Again, see the documentation about how to use them. For example, tokencount(res, "_") will return the total number of sentences. And tokenat(res, 2, "_") will return the third sentence from the res string.
DialogueText Set Text to tokenat(res, sentenceNumber, "_")