Unfortunately, there is no way to do this out-of-the-box with 3CX. My assumption is that you would define "available" as (1) logged into the queue, (2) presence set to available, and (3) at least one phone registered. Further, I would also assume if someone is "available" but currently on the phone, that would not require notification.
It would be possible to monitor each queue using a CFD application and check the agent status. The challenge with this approach is someone would have to remember to start the CFD after each 3CX reboot. Or you could use the 3CX Call Control API and a service to monitor agents.
Either approach could send notifications (email, SMS text, overhead paging, and phone calls).