this.getStyle().numberFormat="#,##,##,##0.000";

var columnvalue=row["value"], customformat="#,##,##,##0.000"; //we can add here a test for conditional formatting
if (columnvalue!=null){
  var symbols=Packages.com.ibm.icu.text.DecimalFormatSymbols(new Packages.java.util.Locale("en","IN"));
  var formatter=Packages.com.ibm.icu.text.DecimalFormat(customformat,symbols);
  var value=new Packages.java.math.BigDecimal(columnvalue.toString());
formatter.format(value);
}else{
  "-"
}

Locale locale = new Locale("en","IN");
String str = NumberFormat.getNumberInstance(locale).format(<your number>);