C++ :

#include <stdio.h>
#include <math.h>
#define MIN_ERR 1e-4
double a,b,c,d;
double f(double x)
{
return a*x*x*x+b*x*x+c*x+d;
}
double find_sol(double min,double max)
{
if (max-min<MIN_ERR) return (min+max)/2;
else
{
double f1,f2,f3;
f1=f(min);
f2=f((min+max)/2);
f3=f(max);
if (f2==0) return (min+max)/2;
else if (f1*f2<0) return find_sol(min,(min+max)/2);
else return find_sol((min+max)/2,max);
}
}
main()
{
double x1,x2;
scanf("%lf%lf%lf%lf",&a,&b,&c,&d);
if (a<0)
{
a=-a;
b=-b;
c=-c;
d=-d;
}
x1=(-2*b-sqrt(4*b*b-12*a*c))/(a*6);
x2=(-2*b+sqrt(4*b*b-12*a*c))/(a*6);
printf("%.2lf %.2lf %.2lf\n",find_sol(-101,x1),find_sol(x1,x2),find_sol(x2,101));
}

Java :

import java.util.Scanner;

public class Main {
	static double m[] = new double[3];
	static int n=0;
	static boolean f(int a, int b, int c, int d) {
		for(double i=-100;i<=100;i++) {
			if(a*i*i*i+b*i*i+c*i+d==0) {
				m[n]=i;
				n++;
			}
		}
		if(Math.abs(m[0]-m[1])>=1 && Math.abs(m[0]-m[2])>=1 && Math.abs(m[1]-m[2])>=1) {
			return true;
		} else return false;
		
	}

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int d = sc.nextInt();
		if(f(a,b,c,d)) {
			System.out.printf("%.2f ",m[0]);
			System.out.printf("%.2f ",m[1]);
			System.out.printf("%.2f ",m[2]);
			}
		sc.close();

	}

}