If your three lines are not in the string buffer you are matching against already, you cannot solve this with regular expressions. I would maintain a "skip" count to know how many lines to skip:

my $skip;
while (<>) {
    next
        if $skip--;
    if( /mypattern/ ) {
        $skip= 3;
    } else {
        print "Have line '$_'";
    };
};
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If your three lines are in the string buffer already, a new line is defined by the newline character(s) appearing. So you would match three newline charachters with non-newline characters in between them to skip three lines forward.

Without seeing code, it's hard to advise better.