A variation of this is answered in "Re: Reading multiple lines?". Maybe that technique could be of use.

For example, cutting and pasting and altering slightly:

my @buffer; 
{ 
  push @buffer, scalar <IN>; 
  redo unless eof(IN) or @buffer >= 10; 
  ## process @buffer 
  shift @buffer;## [was:] @buffer = (); 
  redo unless eof(IN); 
}
[download]

Change the 10 to 3 and you're nearly done.

-- Randal L. Schwartz, Perl hacker