This is a fundamental problem with Webhooks - in the current implementation you can't chain Webhooks together in a reliable way and because they are rate limited you can't even guarantee the initial Webhook will fire in the first place.
But there is an easy solution using script. If you use the API to mark task A as complete you can further process task B and C only after task A's processing is completed. This property of chaining together asynchronous processes is called "thenable". Webhooks are not "thenable" in their current implementation but there are simple ways to make API calls "thenable" (ie chainable) using jQuery or the Fetch API.
