Are you seeing "preconnect " in your Data Breakdown? Preconnect is our 3G Mobile Broadband access point name (APN). The APN acts as your gateway between the Optus Mobile Network and the wider internet.
Rather than having 3 separate Mobile Broadband plans, have you considered something along the lines of a Wireless Home Broadband service? We have 200GB and 500GB data options.
In saying that, there is a drawback. The service can only be used at home and not on the go (whereas Mobile Broadband plans are considered are a portable alternative. You can read more about the differences here.
In this situation being on a public forum we'll really need you to help us help you by telling us how your service was used at that time?
How many people were using the internet? What were they doing? Was there a software update on a device?