Python-сообщество

         combo_3 = QComboBox(self)     #  <—— заменить combo_3 на self.combo_3
        combo_3.move(100, 90)         #  <—— заменить combo_3 на self.combo_3
        combo_3.resize (130, 22)      #  <—— заменить combo_3 на self.combo_3
        combo_3.activated[str].connect(self.onActivated3)   #  <—— заменить combo_3 на self.combo_3

 for row in combo3list:
            self.combo_3.addItem(row)

 import sys
import sqlite3
from PyQt5.QtWidgets import *
from PyQt5.QtGui import QIcon, QFont
from PyQt5.QtCore import QCoreApplication
 
 
class MyWin(QWidget):
    def __init__(self):
        super().__init__()
        self.initUI()
    def initUI(self):
        #comboBox1
        self.lb8 = QLabel("ComboBox_1", self)
        self.lb8.move(30, 50)
 
        self.lbl = QLabel(self)
        self.lbl.move(250, 52)
 
        combo = QComboBox(self)
        combo.move(110, 50)
        combo.addItems(["","Raion1", "Raion2","Raion3","Raion4","Raion5"])
        combo.resize (130, 22)
        combo.activated[str].connect(self.onActivated)
        combo.activated[str].connect(self.selected)
        combo.activated[str].connect(self.combo3)
 
        #comboBox3
        self.lb10 = QLabel("ComboBox_3", self)
        self.lb10.move(30, 90)
 
        self.lb3 = QLabel(self)
        self.lb3.move(250, 90)
 
        self.combo_3 = QComboBox(self)
        self.combo_3.move(110, 90)
        self.combo_3.resize (130, 22)
        self.combo_3.activated[str].connect(self.onActivated3)
 
        #Close button
        qbtn = QPushButton('Close', self)
        qbtn.clicked.connect(self.close)
        qbtn.resize(qbtn.sizeHint())
        qbtn.move(450, 450)
 
        #Basic Window
        self.resize(600, 520)
        self.center()
        self.setWindowTitle('Center')
        self.setWindowTitle('Locality')
        self.setWindowIcon(QIcon('port.png'))
        self.show()
 
        #qLabel_1
    def onActivated(self, text):
        self.lbl.setText(text)
        self.lbl.adjustSize()
 
        #qLabel_3
    def onActivated3(self, text):
        self.lb3.setText(text)
        self.lb3.adjustSize()
 
        #selected raion
    def selected(self, text):
        raion = text
 
    def combo3(self, text):
        conn = sqlite3.connect ("Locality.db")
        cursor = conn.cursor()
        cursor.execute ("""select locality from Locality where raion=?""",(text,))
        result = cursor.fetchall()
        conn.close()                      #добавил закрытие
        self.combo_3.clear()              #добавил очистку виджета
        combo3list = [row[0] for row in result]
        print('list:', combo3list)
        for row in combo3list:
            self.combo_3.addItem(row)
 
        #main window
    def center(self):
        qr = self.frameGeometry()
        cp = QDesktopWidget().availableGeometry().center()
        qr.moveCenter(cp)
        self.move(qr.topLeft())
 
    def closeEvent(self, event):
        reply = QMessageBox.question(self, 'Message', "Are you sure to quit?",
                                     QMessageBox.Yes | QMessageBox.No, QMessageBox.No)
        if reply == QMessageBox.Yes:
            event.accept()
        else:
            event.ignore()
 
if __name__ == '__main__':
    app = QApplication(sys.argv)
    my = MyWin()
    sys.exit(app.exec_())