Graphics Effects don't work properly when are set both to widget and its children

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
   this->resize(120, 115);

    QFrame *frm = new QFrame(this);
    frm->setGeometry(this->geometry());

    QPushButton *btn1 = new QPushButton("Btn1", frm);
    btn1->setGeometry(10, 10, 100, 25);

    QPushButton *btn2 = new QPushButton("Btn2", frm);
    btn2->setGeometry(10, 45, 100, 25);

    QPushButton *btn3 = new QPushButton("Btn3", frm);
    btn3->setGeometry(10, 80, 100, 25);

    {
        QGraphicsDropShadowEffect *effect = new QGraphicsDropShadowEffect(btn1);
        effect->setBlurRadius(30);
        effect->setColor(Qt::black);
        effect->setOffset(0,0);
        btn1->setGraphicsEffect(effect);
    }

    {
        QGraphicsDropShadowEffect *effect = new QGraphicsDropShadowEffect(btn3);
        effect->setBlurRadius(30);
        effect->setColor(Qt::black);
        effect->setOffset(0,0);
        btn3->setGraphicsEffect(effect);
    }

    {
        QGraphicsBlurEffect *effect = new QGraphicsBlurEffect(frm);
        effect->setBlurRadius(2);
        frm->setGraphicsEffect(effect);
    }

}

MainWindow::~MainWindow()
{

}