Dude,
1. For this first you need to keep track of the time for which any customer is booking a table. You can keep the duration time of 3 hours after which the reservation will fail.
2. Second thing, maintain all this in database. Keep updating the database.
3. I will not suggest you the use of internal database of ASP.NET. Instead try using the SQL SERVER 2008 R2 version. It has got the solutions that u need.
Regards
Market Research