单击按钮后向屏幕添加框架

不要更改 Qt Designer 生成的代码。创建另一个从相应小部件继承的类，并使用生成的类来填充它。

无效 QWidget::显示（）

显示小部件及其子小部件。

from PyQt5 import QtCore, QtGui, QtWidgets

font = QtGui.QFont()
font.setFamily('Montserrat')
font.setPointSize(12)
font.setBold(True)
font.setWeight(75)

class Ui_Window3(object):
    def setupUi(self, Window3):
        Window3.setObjectName('Window3')
        Window3.resize(450, 336)
        Window3.setWindowTitle(' ')
        icon = QtGui.QIcon()
        icon.addPixmap(QtGui.QPixmap('icon.png'), QtGui.QIcon.Normal, QtGui.QIcon.Off)
        Window3.setWindowIcon(icon)
        Window3.setStyleSheet('background-color: #fefff7')
        self.Main3 = QtWidgets.QWidget(Window3)
        self.Main3.setObjectName('Main3')

        self.but_5 = QtWidgets.QPushButton(self.Main3)
        self.but_5.setGeometry(QtCore.QRect(0, 276, 450, 60))
        self.but_5.setFont(font)
        self.but_5.setStyleSheet('QPushButton {color: #000000; background-color: #c7dcd0; border-radius: 0;} QPushButton: pressed {background-color: #9babb2;}')
        self.but_5.setObjectName('but_5')

        Window3.setCentralWidget(self.Main3)

        self.retranslateUi(Window3)
        QtCore.QMetaObject.connectSlotsByName(Window3)

    def retranslateUi(self, Window3):
        _translate = QtCore.QCoreApplication.translate
        self.but_5.setText(_translate('Window3', 'КНОПКА'))


class MainWindow(QtWidgets.QMainWindow, Ui_Window3):
    def __init__(self):
        super().__init__()
        self.setupUi(self)

        self.but_5.clicked.connect(self.results)

    def results(self):
        #рамка которую нужно добавить на экран после нажатие на кнопку
        self.text_3 = QtWidgets.QLabel(self.Main3)
        self.text_3.setGeometry(QtCore.QRect(0, 138, 450, 60))
        self.text_3.setFont(font)
        self.text_3.setStyleSheet('''
            background-color: #c7dcd0; 
            border:2px solid #9babb2; 
            color: #000000;
        ''')
        self.text_3.setText('РАМКА')
        self.text_3.setAlignment(QtCore.Qt.AlignCenter)
        self.text_3.setObjectName('text_3')
        self.text_3.show()                                         # +++ 
        
        
if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())