Or, have N available 3 hour windows, and if you've interacted (viewed the website/posted) in any way with the website during one of those periods, you cannot us

Or, have N available 3 hour windows, and if you've interacted (viewed the website/posted) in any way with the website during one of those periods, you cannot use the other periods for that day. So basically the same idea, but letting the decision be more dynamic.