If you don't mind changing your structure a bit, you can do that:
"Select1"[00]"Select2"[00]"Select3"[00]→Str0
For(I,0,2)
Text(0,6*I,stdDev(Str0,I))
End
This works even if your part strings are not the same length (like "A"[00]"BBBBB"[00]"CC"[00]).
(Runer112 pointed out on IRC that you can just use your code with my data and forget about stdDev if you keep them the same length).

If you do mind about keeping your structure (with spaces), you can just use a Copy before Text:
For(I,0,2)
Copy(I*8+Str0,L1,8)
Text(0,6*I,L1)
End

Now, about basic optimizations, you can write I*6 instead of 6*I and write your loop without an official loop variable (like a For(3)).