I'm not sure how to do this exactly with a regex but one could use something like: -

use strict;

my @names = (
    'Joe John Smith',
    'David George',
    'Emma Harry Sally Martin',
);

foreach (@names) {
    my @w = split /\s/, (/\s+(\w[\w ]{0,18}\w)\s*$/)[0];
    shift @w while @w > 2;
    print "@w\n";
}
[download]

Update: You can replace everything in the foreach scope with: -

(/\s+(\w[\w ]{0,18}\w)\s*$/)[0] =~ /(\w+\s?\w+)$/g;
[download]