#!/usr/bin/perl -w
use strict;
my $delimiter = ';';
my $nth = 3;
my $regex = '^(' . ".*$delimiter"x$nth . ')';

while (<DATA>) {
  s/$regex/$1$delimiter/ && print;
}

__DATA__
col1;col2;col3;col4
aaa;bbb;ccc;ddd
AAA;BBB;CCC;DDD