java - Adding A New Line Every Three Digits -

okay, trying add new line every 3 letters. goal have 20 rows of randomized capital letters. 3 capital letters per row.

i randomized numbers , converted them string. outputted string. have 60 random capital letters in 1 straight string, can't figure out how add new lines, since each letter randomized can't tell java search did before. have tried using if statement , add "/n" @ end, got messy , difficult. , operator commands couldn't figure out how apply every 3 letters either.

here code have far under "wrightrandomcodestofile"

package mp05;  import java.io.file; import java.io.filenotfoundexception; import java.io.printwriter; import java.util.random; import java.util.scanner;  public class main {     private static final int product_count = 20;     private static final int characters_per_code = 3;     private static int chara;     private static int prechars;     private static string preletters;      public static void main(string[] args) {         /* (1) generate 3 character alphabetic prefix codes. */         writerandomcodestofile("prefix.txt", 'a', 'z',                               characters_per_code, product_count);         // when hit run, there 3 numbers. 6 digits though.          /* (2) generate 3 character alphabetic suffix codes. */         writerandomcodestofile("suffix.txt", 'a', 'z',                                characters_per_code, product_count);          /* (3) generate 3 character numeric inline codes. */         writerandomcodestofile("inline.txt", '0', '9',                                characters_per_code,  product_count);          /* (4) merge 2 alphabetic , 1 numeric code produce          * final product code. */         mergeproductcodestofile("prefix.txt", "inline.txt", "suffix.txt",                                 "productcode.txt");     }// end main()       /* (1),(2),(3) generate alphanumeric codes. these 3 aphanumeric      * character combinations generated randomly each character in      * range a-z | 0-9. have codes aaa-zzz | 000-999.      *      * write numberofcodestogenerate codes specified file.      */     public static void writerandomcodestofile(string codefile,                                               char fromchar, char tochar,                                               int numberofcharacterspercode,                                               int numberofcodestogenerate) {         random rnd = new random();         chara = 65;         (int = 1; <= 20; i++) {             try (                 printwriter fout = new printwriter(new file("prefix.txt")); //creates file             ) {                 prechars = chara + rnd.nextint(26);                 preletters = string.valueof((char) prechars);                 system.out.print(preletters);                  } catch (filenotfoundexception e) {                  system.err.println(e.getmessage());             }         }     }// end writerandomcodestofile()      /* (4) merge 2 alphabetic , 1 numeric code produce final      * product code. final product code consist of <prefix><inline><suffix>.      * each code in range aaa000aaa-zzz999zzz.      *      * merge using prefix, inline , suffix code corresponding      * files. each individual code (pre, in, post) used once, , values used.      *      * write these final product codes file specified in productfile.      */     public static void mergeproductcodestofile(string prefixfile,                                                string inlinefile,                                                string suffixfile,                                                string productfile) {       }// end mergeproductcodestofile() }// end main 

ignoring context , answering question per title:

given long string, insert newline every 3 chars:

str = str.replaceall("...", "$0\n");