Virtual Function Pointer Table

[humble_learner]

#include <iostream>
using namespace std;

class Base1 {
public:
	virtual void f() { }
};

class Base2 {
public:
	virtual void f() { }
};

class Base3 {
public:
	virtual void f() { }
};

class Drive : public Base1, public Base2, public Base3 {
};

int main() {
	Drive objDrive;
	cout << "Size is = " << sizeof(objDrive) << endl;
	return 0;
}

Why is the sizeof(objDrive) being indicated as 12 ?

I understand that :
4 bytes for virtual fptr table for Base 1, 4 bytes for virtual fptr table for Base 2,4 bytes for virtual fptr table for Base 3.

What about 4 bytes for virtual fptr table for Drive ?