C :

#include<stdio.h>
#include<math.h>
int main()
{
    float a,b,c,d;
    scanf("%f%f%f",&a,&b,&c);
    d=(a+b+c)/2.;
    d=sqrt(d*(d-a)*(d-b)*(d-c));
    printf("%.3f",d);
}

C++ :

#include<stdio.h>
#include<math.h> 
int main()
{
    double a,b,c,s,p;
    while(scanf("%lf %lf %lf",&a,&b,&c)!=EOF)
    {
        p=(a+b+c)/2.0000;
        p=p*(p-a)*(p-b)*(p-c);
        s=sqrt(p); 
        printf("%.3lf\n",s); 
    } 
} 

Java :

import java.util.Scanner;

public class Main {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		
		Scanner input = new Scanner(System.in);
		
		double a = input.nextDouble();
		double b = input.nextDouble();
		double c = input.nextDouble();
		double s = (a + b + c)/2.0;
		double area = Math.sqrt(s*(s-a)*(s-b)*(s-c));
		System.out.printf("%.3f",area);
		input.close();
	}
	
	

}

Python :

a = map(lambda x:float(x), raw_input().split())
x = sum(a)/2

print "%.3f"%((x*(x-a[0])*(x-a[1])*(x-a[2]))**0.5)