Is there a specific time frame that airlines open up their business class seats? I am looking at the US airways flight US796 on Dec. 4th from PHL-TLV...When I search for two tickets, it shows that it is available. When I search for 3, it doesn't. But on the US site, all of them are free except one (I need 3). Is it possible to book those two and another opens up? Is it arbitrary or is there a specific guideline they follow?