Hi
First question, are the text fields in the repeating table? Second question, if they are, are you trying to display the concat outside of the repeating table?
Thirdly, I think what you are asking is the you have a single Field in a repeating table and you are looking to concatenate all the instances of field 1 in the form
"Field1a", ", " "Field1b", " and ", "Field1c"
Natively, InfoPath does not support this funcitonality and I am sorry to say you will need code to achieve this.
Hopefully it is not that compicated
If it isn't that complicated then variant field will either introduce extra commas or spaces if they are not populated, just add all the field in with ", " between them and it will work fine. You might want to consider the spaces then it would look slightly less obvious