One thing to note is to convert one of the multipliers to long long in advance so that the result in the middle won’t be truncated to int because of overflow.

#include<bits/stdc++.h>

using namespace std;
//Implement the class Box  
//l,b,h are integers representing the dimensions of the box

// The class should have the following functions : 

// Constructors: 
// Box();
// Box(int,int,int);
// Box(Box);


// int getLength(); // Return box's length
// int getBreadth (); // Return box's breadth
// int getHeight ();  //Return box's height
// long long CalculateVolume(); // Return the volume of the box

//Overload operator < as specified
//bool operator<(Box& b)

//Overload operator << as specified
//ostream& operator<<(ostream& out, Box& B)

class Box
{
    friend ostream& operator<<(ostream& out, const Box& B)
    {
        out << B.l << " " << B.b << " " << B.h;
        return out;
    }

private:
    int l, b, h;

public:
    Box()
        :
        l(0),
        b(0),
        h(0)
    {}

    Box(int l, int b, int h)
        :
        l(l),
        b(b),
        h(h)
    {}

    Box(const Box& box)
    {
        l = box.l;
        b = box.b;
        h = box.h;
    }

    int getLength()
    {
        return l;
    }

    int getBreadth()
    {
        return b;
    }

    int getHeight()
    {
        return h;
    }

    long long CalculateVolume()
    {
        long long l_l = l;
        return l_l * b * h;
    }

    bool operator<(const Box& b)
    {
        if(this->l < b.l) return true;
        if(this->b < b.b && this->l == b.l) return true;
        if(this->h < b.h && this->b == b.b && this->l == b.l) return true;
        return false;
    }

    

};




void check2()
{
	int n;
	cin>>n;
	Box temp;
	for(int i=0;i<n;i++)
	{
		int type;
		cin>>type;
		if(type ==1)
		{
			cout<<temp<<endl;
		}
		if(type == 2)
		{
			int l,b,h;
			cin>>l>>b>>h;
			Box NewBox(l,b,h);
			temp=NewBox;
			cout<<temp<<endl;
		}
		if(type==3)
		{
			int l,b,h;
			cin>>l>>b>>h;
			Box NewBox(l,b,h);
			if(NewBox<temp)
			{
				cout<<"Lesser\n";
			}
			else
			{
				cout<<"Greater\n";
			}
		}
		if(type==4)
		{
			cout<<temp.CalculateVolume()<<endl;
		}
		if(type==5)
		{
			Box NewBox(temp);
			cout<<NewBox<<endl;
		}

	}
}

int main()
{
	check2();
}

References

1. https://en.cppreference.com/w/cpp/language/operators
2. https://en.cppreference.com/w/cpp/language/friend