find the sum of the multiples of 3 and 5 below 1000

you should use the same for loop for both to aviod double counting numbers that are multiple of both. such as 15,30...

   for(int temp =0; temp < 1000 ; temp++){
        if(temp % 3 == 0){
            x.add(temp);
            totalforthree += temp;
        }else if(temp % 5 == 0){
            y.add(temp);
            totalforfive += temp;
        }
    }

In a mathematical perspective,
You did not consider about common factors between 3 and 5.
Because there is double counting.

ex; number 15 , 30 , 45 , 60 , 75 , 90 , 105 , 120 , 135 , 150 , 165 , 180 , 195 , 210 , 225 , 240 , 255 , 270 , 285 , 300 , 315 , 330 , 345 , 360 , 375 , 390 , 405 , 420 , 435 , 450 , 465 , 480 , 495 , 510 , 525 , 540 , 555 , 570 , 585 , 600 , 615 , 630 , 645 , 660 , 675 , 690 , 705 , 720 , 735 , 750 , 765 , 780 , 795 , 810 , 825 , 840 , 855 , 870 , 885 , 900 , 915 , 930 , 945 , 960 , 975 , 990 , are common factors.

total of common factors = 33165.
Your answer is 266333
Correct answer is 233168.
Your Answer - Total of common factors
266333-33165=233168.

(this is a code for getting common factors and Total of common factors )

public static void main(String[] args) {

System.out.println("The sum of the Common Factors : " + getCommonFactorSum());

}

private static int getCommonFactorSum() {
int sum = 0;
for (int i = 1; i < 1000; i++) {
    if (i % 3 == 0 && i % 5 == 0) {
        sum += i;
        System.out.println(i);
    }
}