<?php
add_filter( 'woocommerce_checkout_fields', function ( $fields ) {
	$offset            = 2; //Change this by the field position you want to add (remembering it starts from zero)
	$custom_field_id   = 'billing_wcj_checkout_field_1'; //Your custom field id
	$oldArray          = $fields['billing'];
	$custom_field      = array( $custom_field_id => $oldArray[ $custom_field_id ] );
	$newArray          = array_slice( $oldArray, 0, $offset, true ) + $custom_field + array_slice( $oldArray, $offset, null, true );
	$fields['billing'] = $newArray;
	return $fields;
}, PHP_INT_MAX );