#!/usr/bin/perl
use strict;
use warnings;

my %mapping;
open my $key_handle, '<', 'file2.txt' or die "Open fail: $!";
while (<$key_handle>) {
    my ($key, $value) = split;
    $mapping{$key} = $value;
}

open my $data_handle, '<', 'file1.txt' or die "Open fail: $!";
open my $out_handle, '>', 'file3.txt' or die "Open fail: $!";

while (<$data_handle>) {
    my @columns = split;
    print $out_handle "$columns[0]\t$columns[1]\t$mapping{$columns[2]}\n"
}