To write a Python code using PyQt for an application window with three tabs, and each tab having three buttons, we need to follow the steps outlined below: Step 1: Import the required PyQt modules We need to import the necessary PyQt modules to create GUI elements and to display the window.

from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *

Step 2: Create the main window We need to create a QMainWindow object to serve as the main window. This window will contain the tab widget.

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setWindowTitle('My Application')
        self.setGeometry(100, 100, 600, 400)
        self.tab_widget = QTabWidget(self)
        self.setCentralWidget(self.tab_widget)

Step 3: Create the tab widgets Next, we need to create the tab widgets and add them to the main window. We will create three tabs - "Tab 1", "Tab 2", and "Tab 3".

        self.tab_1 = QWidget()
        self.tab_2 = QWidget()
        self.tab_3 = QWidget()
        
        self.tab_widget.addTab(self.tab_1, "Tab 1")
        self.tab_widget.addTab(self.tab_2, "Tab 2")
        self.tab_widget.addTab(self.tab_3, "Tab 3")

Step 4: Create buttons for each tab Now, we need to create three buttons for each tab. We will add the buttons to QVBoxLayout objects, which will be used as layouts for each tab.

        # Tab 1
        self.tab_1_layout = QVBoxLayout()
        self.tab_1_button_1 = QPushButton('Button 1', self.tab_1)
        self.tab_1_button_2 = QPushButton('Button 2', self.tab_1)
        self.tab_1_button_3 = QPushButton('Button 3', self.tab_1)
        self.tab_1_layout.addWidget(self.tab_1_button_1)
        self.tab_1_layout.addWidget(self.tab_1_button_2)
        self.tab_1_layout.addWidget(self.tab_1_button_3)
        self.tab_1.setLayout(self.tab_1_layout)

        # Tab 2
        self.tab_2_layout = QVBoxLayout()
        self.tab_2_button_1 = QPushButton('Button 1', self.tab_2)
        self.tab_2_button_2 = QPushButton('Button 2', self.tab_2)
        self.tab_2_button_3 = QPushButton('Button 3', self.tab_2)
        self.tab_2_layout.addWidget(self.tab_2_button_1)
        self.tab_2_layout.addWidget(self.tab_2_button_2)
        self.tab_2_layout.addWidget(self.tab_2_button_3)
        self.tab_2.setLayout(self.tab_2_layout)

        # Tab 3
        self.tab_3_layout = QVBoxLayout()
        self.tab_3_button_1 = QPushButton('Button 1', self.tab_3)
        self.tab_3_button_2 = QPushButton('Button 2', self.tab_3)
        self.tab_3_button_3 = QPushButton('Button 3', self.tab_3)
        self.tab_3_layout.addWidget(self.tab_3_button_1)
        self.tab_3_layout.addWidget(self.tab_3_button_2)
        self.tab_3_layout.addWidget(self.tab_3_button_3)
        self.tab_3.setLayout(self.tab_3_layout)

Step 5: Run the application Finally, we need to run the application by creating an instance of the MainWindow class and calling the app.exec_() method.

if __name__ == '__main__':
    import sys
    app = QApplication(sys.argv)
    main_window = MainWindow()
    main_window.show()
    sys.exit(app.exec_())

The complete Python code using PyQt for an application window with three tabs, and each tab having three buttons is as follows:

from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setWindowTitle('My Application')
        self.setGeometry(100, 100, 600, 400)
        self.tab_widget = QTabWidget(self)
        self.setCentralWidget(self.tab_widget)

        self.tab_1 = QWidget()
        self.tab_2 = QWidget()
        self.tab_3 = QWidget()
        
        self.tab_widget.addTab(self.tab_1, "Tab 1")
        self.tab_widget.addTab(self.tab_2, "Tab 2")
        self.tab_widget.addTab(self.tab_3, "Tab 3")

        # Tab 1
        self.tab_1_layout = QVBoxLayout()
        self.tab_1_button_1 = QPushButton('Button 1', self.tab_1)
        self.tab_1_button_2 = QPushButton('Button 2', self.tab_1)
        self.tab_1_button_3 = QPushButton('Button 3', self.tab_1)
        self.tab_1_layout.addWidget(self.tab_1_button_1)
        self.tab_1_layout.addWidget(self.tab_1_button_2)
        self.tab_1_layout.addWidget(self.tab_1_button_3)
        self.tab_1.setLayout(self.tab_1_layout)

        # Tab 2
        self.tab_2_layout = QVBoxLayout()
        self.tab_2_button_1 = QPushButton('Button 1', self.tab_2)
        self.tab_2_button_2 = QPushButton('Button 2', self.tab_2)
        self.tab_2_button_3 = QPushButton('Button 3', self.tab_2)
        self.tab_2_layout.addWidget(self.tab_2_button_1)
        self.tab_2_layout.addWidget(self.tab_2_button_2)
        self.tab_2_layout.addWidget(self.tab_2_button_3)
        self.tab_2.setLayout(self.tab_2_layout)

        # Tab 3
        self.tab_3_layout = QVBoxLayout()
        self.tab_3_button_1 = QPushButton('Button 1', self.tab_3)
        self.tab_3_button_2 = QPushButton('Button 2', self.tab_3)
        self.tab_3_button_3 = QPushButton('Button 3', self.tab_3)
        self.tab_3_layout.addWidget(self.tab_3_button_1)
        self.tab_3_layout.addWidget(self.tab_3_button_2)
        self.tab_3_layout.addWidget(self.tab_3_button_3)
        self.tab_3.setLayout(self.tab_3_layout)

if __name__ == '__main__':
    import sys
    app = QApplication(sys.argv)
    main_window = MainWindow()
    main_window.show()
    sys.exit(app.exec_())

This code will create an application window with three tabs, and each tab having three buttons.