template <typename T, typename, int, template <typename U, U, U> class>
struct Sort;
This is perfectly legal.
It could be redeclared like this, giving names to all the parameters:
template <typename T, typename T2, int I, template <typename U, U X, U Y> class TT>
struct Sort;
It declares a class template Sort which has four template parameters, the type parameter T, a second type parameter T2 (unnamed in the original), a non-type template parameter I, and a template template parameter TT.
The template template parameter TT must a class template taking three template parameters, U is a type parameter and the second and third (X and Y) are non-type template parameters of type U.
A suitable argument for the fourth template parameter of Sort might be something like:
template <typename T, T t1, T t2>
class Foo
{ static const bool value = t1 < t2; };
which would be instantiated like:
Foo<int, 1, 2> fi;
or
Foo<char, 'a', 'b'> fc;