And if the OP ever had the need to scale it up to more than 3 characters: 
use strict;
use warnings;

my $n = 3; # number of adjacent characters
$n--;

$_ = 'testing 1234567 ........ aaaaaaaasssssss __________ ++++++++++ -
+-------- testing testing ';

s/((.)\2{$n})\2*/$1/g;
print;

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In reply to Re^2: Supress similar chars in the string by toolic
in thread Supress similar chars in the string by Lana

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