Selecting random posts from an array

On my blog homepage I’m using a wp query to display 8 featured posts from an array. Now I’m trying to write a script that would randomly pick 3 elements out of the first 5 elements of the array and assign them a custom css class (.feat_large) and a specific post thumbnail the_post_thumbnail( ‘homepage-thumb-large’ ). Remaining 5 posts should use the the_post_thumbnail( ‘homepage-thumb-small’ ) and no specific css class.

I managed to achieve something similar in JS, but now I’d like to have a server-side php solution. My knowledge of php is very limited, so I’d appreciate some help on this issue.