If I still were a Drupal core developer (and I was the #1 core contributor for 5, 6 and #4 for 7 and still #10 for 8 though I regret that) I would tell you there's no tough choice here, I would target PHP 7.x and forget the rest. Currently, others steers the core and they struggle mightily with just the transition to PHP 7.0 requirement. Check the issue queue. I filed https://www.drupal.org/node/2605226 in 2015 fall but it was voted down, the current open issue is https://www.drupal.org/node/2842431

HHVM is way too niche to bother with it, sorry.