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Your method calls are wrong, they must be
double smResult = smallest(x,y,z);
double avgResult = average(x,y,z);

That might be because the 'if' is not having an ending '}'

Firstly, the 'main' method declaration is wrong. It has to be public static void main. You missed void

--- Update ---

else should not have condition, else (z < x && z < y)
{return z;
}

It...