This would be a good use of natatime

#!/usr/bin/env perl 
use strict;
use warnings;
use feature 'say';
use List::MoreUtils 'natatime';

my $string = "firstname1:::surname1:::middlename1:::firstname2:::surna
+me2:::middlename2:::firstname3:::surname3:::middlename3";
my @arr;

my $it = natatime (3, split (/:::/, $string));
while (my @these = $it->()) {
    push @arr, join ':::', @these;
}

say for @arr;
[download]