How can I show 3 entries at random from within the next 10 {% for entry in craft.entries.section('entertainmentEvents').status('live','pending').search('entertainmentBanner:*').order('RAND()').limit(3) %}


This is working to show 3 random entries from the full list of all entries with the entertainmentBanner field populated. How would I limit it to the first ten entries by entry date and then 3 at random from within them?
You could use Craft's shuffle() function to randomize your postDate ordered array of entry models:

{% set entries = craft.entries({
    section: 'entertainmentEvents',
    entertainmentBanner: ':notempty:',
    status: 'live, pending',
    order: 'postDate desc',
    limit: 10
}) %}

{% set entries = shuffle(entries) %}

{% for entry in entries|slice(0, 3) %}
    ...
{% endfor %}