How can I combine meta_query queries?

Using your constructed array, I think this is the correct syntax:

$meta_query_args = array(
    'meta_key' => 'price',
    'meta_value_num' => 1000,
    'meta_compare' => '<=',
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key' => 'roomcount',
            'value' => 3,
            'type' => 'numeric',
            'compare' => '>='
        ),
        array(
            'key' => 'bathroomcount',
            'value' => 2,
            'type' => 'numeric',
            'compare' => '>='
        )
    )
);

$realestate_meta_query = new WP_Query( $meta_query_args );

You might also try nesting meta_query one more level (though I don’t know if that will work):

$meta_query_args = array(
    'meta_query' => array(
        'relation' => 'AND',
        array(
            'key' => 'price',
            'value' => 1000,
            'type' => 'numeric',
            'compare' => '<='
        ),
        array(
            'relation' => 'OR',
            array(
                'key' => 'roomcount',
                'value' => 3,
                'type' => 'numeric',
                'compare' => '>='
            ),
            array(
                'key' => 'bathroomcount',
                'value' => 2,
                'type' => 'numeric',
                'compare' => '>='
            )
        )
    )
);

$realestate_meta_query = new WP_Query( $meta_query_args );