Java O(n)

public static int equalStacks(List<Integer> h1, List<Integer> h2, List<Integer> h3) {
    int l1=0,l2=0,l3=0;

    Stack<Integer> h11= new Stack<Integer>();
    Stack<Integer> h12= new Stack<Integer>();
    Stack<Integer> h13= new Stack<Integer>();

    for(int i=h1.size()-1;i>=0;i--){
        l1+=h1.get(i);
        h11.push(h1.get(i));
    }
    for(int i=h2.size()-1;i>=0;i--){
        l2+=h2.get(i);
        h12.push(h2.get(i));
    }
    for(int i=h3.size()-1;i>=0;i--){
        l3+=h3.get(i);
        h13.push(h3.get(i));
    }

    while( l1!=l2 || l2!=l3){

        if(l1>l2 || l1>l3){
            l1-=h11.pop();
        }
        if(l2>l1 || l2>l3) {
            l2 -= h12.pop();
        }
        if(l3>l2 || l3>l1){
            l3-=h13.pop();
        }
    }
    return l1;
}