I assume your example contains generic field names, but you could use:

  my $string = 'Field1:  one';
  print "$1\n" if $string =~ /(?:Field1:|Field2:|Field3:)\s+(\w{3})/;
[download]

Or if those really are your field names:

  my $string = 'Field1:  one';
  print "$1\n" if $string =~ /Field\d:\s+(\w{3})/;
[download]

The point being that if the set of regexes is small and static, combine them for simpler code (first example), or for better performance (second example).