Hi Jigar,
I don't see anything in your code that jumps out at me as a problem.
In btnAdvSearch_Click you should be able to get the value from txtSearchOption3.Text
In your page load event you might try commenting out this part:
if (prevTxtSearchFor != null)
{
this.txtSearchOption3.Text = prevTxtSearchFor.Text;
}
as that may be setting it back to empty string
Hope it helps,
Joe