Just a guess...

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11129561
use warnings;

my %hash;

while( <DATA> )
  {
  my ($one, $two, $three) = split /\s{2,}|\n/;
  $hash{$one}{$two}{$three}++;
  }
for my $value ( values %hash )
  {
  $_ = [ sort keys %$_ ] for values %$value;
  }
use Data::Dump 'dd'; dd \%hash;

__DATA__
900000001    Network1    Login
900000002    Network1    2019-11-07T20:31:10.000Z
900000002    Network1    2021-02-19T13:47:25.000Z
900000002    Network1    2021-03-10T13:58:50.000Z
900000003    No Network1    NOT Registered
900000004    Network1    Login
900000005    Network1    Login
900000006    Network1    Login2
900000007    Network1    2021-03-10T17:11:24.000Z
900000007    Network1    2021-02-18T08:34:58.000Z
900000007    Network1    2021-01-25T17:19:44.000Z
500000008    Network1    2021-03-10T17:11:24.000Z
500000008    Network2    Not Used
500000008    Network1    2021-03-11T17:12:42.000Z
500000008    Network3    Not Used
400000008    Network1    2021-01-25T17:19:36.000Z
[download]

Outputs:

{
  400000008 => { Network1 => ["2021-01-25T17:19:36.000Z"] },
  500000008 => {
                 Network1 => ["2021-03-10T17:11:24.000Z", "2021-03-11T
+17:12:42.000Z"],
                 Network2 => ["Not Used"],
                 Network3 => ["Not Used"],
               },
  900000001 => { Network1 => ["Login"] },
  900000002 => {
                 Network1 => [
                   "2019-11-07T20:31:10.000Z",
                   "2021-02-19T13:47:25.000Z",
                   "2021-03-10T13:58:50.000Z",
                 ],
               },
  900000003 => { "No Network1" => ["NOT Registered"] },
  900000004 => { Network1 => ["Login"] },
  900000005 => { Network1 => ["Login"] },
  900000006 => { Network1 => ["Login2"] },
  900000007 => {
                 Network1 => [
                   "2021-01-25T17:19:44.000Z",
                   "2021-02-18T08:34:58.000Z",
                   "2021-03-10T17:11:24.000Z",
                 ],
               },
}
[download]