There’s more than one way to autocomplete orders in WooCommerce 3.0. If the products in an order are all virtual and downloadable, the order will be marked completed once payment is received. Any other order will be marked processing to indicate some manual work is needed on the part of the shop owner to complete the transaction. Most articles I found on this topic use an action hooked to
woocommerce_thankyou or a filter on
woocommerce_payment_complete_order_status. There is another way. A better way.