#!/bin/sh

get_last_n() {
    n="$1"; shift; str="$@"
    
    length=$(echo $str | wc -m)
    pos=$(( length - n ))

    echo $str | cut -c"$pos-"
}

my_str="This is my string."

last_three=$(get_last_n 3 $my_str)

echo $last_three