Check Free 3 Letter Domains

free-3-letter-domains.sh

#!/usr/bin/env bash

array=( a b c d e f g h i j k l m n o p q r s t u v w x y z )

for a in "${array[@]}"
do
	for b in "${array[@]}"
	do
		for c in "${array[@]}"
		do
			domain="$a$b$c.de"
			echo $domain
			nslookup $domain|grep NXDOMAIN >> free.txt
		done
	done
done

or

#!/usr/bin/env bash

# Set this:
tld=.de

for a in {a..z}
do
	for b in {a..z}
	do
		for c in {a..z}
		do
			domain="$a$b$c$tld"
			echo $domain
			nslookup $domain|grep NXDOMAIN >> free.txt
		done
	done
done

You might shorten the script with bash's built-in brace expansion.
For example:

#!/bin/bash

tld='de'
for name in {a..z}{a..z}{a..z}.$tld
do	
  dig +short $name soa | grep -qv "^$" || printf "$name\n" | tee -a free.txt
done

EDIT: and, if you wanted to include digits...

tld='de'
for name in {{a..z},{0..9}}{{a..z},{0..9}}{{a..z},{0..9}}.${tld}
do	
  dig +short $name soa | grep -qv "^$" || printf "$name\n" | tee -a free.txt
done