Saving Data in Main Page for Multiple Pages in VS2010 C#

I have a main page (MainPage.aspx) containg three tabs; each tab displays an aspx page (page1.aspx, page2.aspx, page3.aspx) containing some text box controls. User will be entering data in each of the tabs and there are some required fields in all three pages.
On top of the main page; there is a button "btnSaveData" to save data in the database.
When button "btnSaveData" is clicked; how do I validate if required fields have data and then save in the database?
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