Try this.
I followed the WP_Query docs to build this.
$args = [
'meta_query' => [
'relation' => 'AND',
[
'_stock_status' => [
'key' => '_stock_status',
'compare' => 'EXISTS',
],
'_sale_price' => [
'key' => '_sale_price',
'compare' => 'EXISTS',
],
'_price' => [
'key' => '_price',
'compare' => 'EXISTS',
]
]
],
'orderby' => [
'_stock_status' => 'ASC',
'_sale_price' => 'DESC',
'_price' => 'DESC'
]
];
This will order by stock first, sale price second and price third.
There is no way to order by two or more values simultaneously, at least not that I know of or could find.