you can use this:

 Collections.max(Arrays.asList(1,2,3,4));

or create a function

public static int max(Integer... vals) {
    return Collections.max(Arrays.asList(vals)); 
}

You should know more about java.lang.Math.max:

1. java.lang.Math.max(arg1,arg2) only accepts 2 arguments but you are writing 3 arguments in your code.
2. The 2 arguments should be double,int,long and float but your are writing String arguments in Math.max function. You need to parse them in the required type.

You code will produce compile time error because of above mismatches.

Try following updated code, that will solve your purpose:

import java.lang.Math;
import java.util.Scanner;
public class max {
    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        System.out.println("Please input 3 integers: ");
        int x = Integer.parseInt(keyboard.nextLine());
        int y = Integer.parseInt(keyboard.nextLine());
        int z = Integer.parseInt(keyboard.nextLine());
        int max = Math.max(x,y);
        if(max>y){ //suppose x is max then compare x with z to find max number
            max = Math.max(x,z);    
        }
        else{ //if y is max then compare y with z to find max number
            max = Math.max(y,z);    
        }
        System.out.println("The max of three is: " + max);
    }
} 

You can use a simple loop:

Copypublic Integer max(final Collection<Integer> ints) {
    Integer max = Integer.MIN_VALUE;
    for (Integer integer : ints) {
        max = Math.max(max, integer);
    }
    return max;
}