#!/bin/bash

function format_number () {
        rev <<< $1 | \
        sed 's/\([0-9][0-9][0-9]\)/\1,/g' | rev | sed 's/^,//'
}

format_number 123
format_number 1234
format_number 123456
format_number 1234567
format_number 12345678
format_number 1234567890
format_number 12345678901234567890