Python PyQt5 threading QObject: Cannot create children for a parent that is in a different thread

The problem has nothing to do with the use of QThread or not. The problem is that the GUI elements (for example the QWidget, QTextDocument, etc) are not thread-safe so you should not modify them or create them in a different thread than the main one. To emphasize my initial comment in my solution I will not use QThread but I will continue using threading but I will send the information to the main thread through signals(what if they are thread-safe):

import sys, time
from threading import Thread

from PyQt5.QtCore import pyqtSignal, QObject
from PyQt5.QtWidgets import (
    QApplication,
    QMainWindow,
    QWidget,
    QPlainTextEdit,
    QHBoxLayout,
)


class Worker(QObject):
    messageChanged = pyqtSignal(str)

    def start(self, fn):
        Thread(target=self._execute, args=(fn,), daemon=True).start()

    def _execute(self, fn):
        fn(self)

    def write(self, message):
        self.messageChanged.emit(message)


def run1(worker):
    for i in range(20):
        worker.write(f"{i}\n")
        time.sleep(0.0125)


def run2(worker):
    for i in range(20):
        worker.write(f"{i}\n")
        time.sleep(0.0125)


def run3(worker):
    for i in range(20):
        worker.write(f"{i}\n")
        time.sleep(0.0125)


app = QApplication([sys.argv])
win = QMainWindow()

text_1 = QPlainTextEdit()
text_2 = QPlainTextEdit()
text_3 = QPlainTextEdit()

my_widget = QWidget()
my_widget.layout = QHBoxLayout()
my_widget.layout.addWidget(text_1)
my_widget.layout.addWidget(text_2)
my_widget.layout.addWidget(text_3)
my_widget.setLayout(my_widget.layout)

win.setCentralWidget(my_widget)

worker1 = Worker()
worker1.messageChanged.connect(text_1.appendPlainText)

worker2 = Worker()
worker2.messageChanged.connect(text_2.appendPlainText)

worker3 = Worker()
worker3.messageChanged.connect(text_3.appendPlainText)

worker1.start(run1)
worker2.start(run2)
worker3.start(run3)


win.show()

sys.exit(app.exec_())