It is 3 ( or 6 or some other number, I'm not sure ) posts every 24 hours and not per day.
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Suppose the date and time now is 21st June 2013, 10:00 am .
Current time - 24 hours = 20th June 2013, 10:00 am.
If you have submitted less than 3 posts between 21st June 10:00 amd and 20th June 10:00 am, then you can submit another post now.
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This is what must have happened to you.
You submitted 3 posts at 9:00 pm, 9:01pm and 9:02 pm on 20th June.
You tried to submit a post on 21st June at 10:00 am - which you can't because you already submitted three posts between 20th June 10:00 amd and 21st June, 10:00 am.
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