your logical statement will never be true....
str variable can only be one of the values not all three of them.
if (str == "one" && str=="two" && str=="three")
what you probably want is
if (str == "one")
Image1.ImageUrl = "~/images/book_flat_imgs/AWing/first_floor/A_101_booked.png";
else if (str == "two")
Image2.ImageUrl = "~/images/book_flat_imgs/AWing/first_floor/A_102.png";
else if (str == "three")
Image3.ImageUrl = "~/images/book_flat_imgs/AWing/first_floor/A_103.png";
Or this
if (str == "one" || str=="two" || str== "three")
{
Image1.ImageUrl = "~/images/book_flat_imgs/AWing/first_floor/A_101_booked.png";
Image2.ImageUrl = "~/images/book_flat_imgs/AWing/first_floor/A_102.png";
Image3.ImageUrl = "~/images/book_flat_imgs/AWing/first_floor/A_103.png";
}
read up on c# operators and logical operators in general to understand what your code is doing.
http://msdn.microsoft.com/en-us/library/6a71f45d(v=vs.71).aspx[
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