If possible, I recommend using C++11 or newer which allows you to compute the desired result w/out implementing your own function (std::min). As already pointed out in one of the comments, you can do
T minimum(std::min({x, y, z}));
or
T minimum = std::min({x, y, z});
which stores the minimum of the variables x, y and z in the variable minimum of type T (note that x, y and z must have the same type or have to be implicitly convertible to it). Correspondingly, the same can be done to obtain a maximum: std::max({x, y, z}).