Query for first 3 posts to change the look and feel

You can style the first 3 posts differently without querying them separately, you just need to check where you are within the loop while outputting each post via the built in current_post var.

while( have_posts() ):
    the_post();

    // are we on the first page and outputting one of the first 3 posts?
    if( !is_paged() && $wp_query->current_post < 3 ):
        // output full image, etc..
    else:
        // not first page and not first 3 posts
        // output just thumb, etc..
    endif;

endwhile;