syntax error, unexpected '&&' ??????

What's wrong with this..

if (isset($_SESSION['var1'])) && (isset($_SESSION['var2'])) && (isset($_GET['var3'])){

?? looks fine to me..

Nope, the brackets are incorrect:

if (isset($_SESSION['var1'])) && (isset($_SESSION['var2'])) && (isset($_GET['var3'])){
^

That indicates you are no longer in the if, so you need either an open brace or an expression.
What you want is:

if (isset($_SESSION['var1']) && isset($_SESSION['var2']) && isset($_GET['var3'])){

Or even easier:

if (isset($_SESSION['var1'], $_SESSION['var2'], $_GET['var3']))
{

There the combination indicates all of these need to be set (so same as &&, but cannot be used with || logic).