Hi!
If I understand correctly, you want the results randomly ordered? That could be possible with a small custom code snippet.
Try adding this custom code to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case!
This should only activate when less than 3 meta filters are chosen – so if all three are selected, it should return the ordering by relevance.
The drop-down with checkboxes is much more difficult, I am not even sure if it’s possible without major customizations. Unfortunately we are not available for custom jobs, but I usually suggest this service: wpkraken.io
They are a dedicated team of developers specialized in WordPress, so they should be able to help.