Help with adding commas

I’m currently working with DNA sequences and often times give data that looks like “ACGTCGTGCGTGCGTC…” and goes on for thousands of characters. I’d like to be able to divide up the characters in specific ways so I can apply certain rules, etc. Would there be an easy, fast way of placing commas between certain segments of the DNA (between every third letter for example).

dim arr() as string
dim dnaLen as integer = dna.Len
for i as integer = 1 to dnaLen step 3
  arr.Append dna.Mid( i, 3 )
next i

dna = join( arr, "," )

Thanks again!