import tkinter as tk

window = tk.Tk()
value = tk.StringVar()

entry1 = tk.Entry(window, textvariable=value)
entry2 = tk.Entry(window, textvariable=value)
entry3 = tk.Entry(window)

def send_value():
    entry3.delete(0, tk.END)
    entry3.insert(0, value.get())

button_ok = tk.Button(window, text='OK', command=send_value)

entry1.pack()
entry2.pack()
entry3.pack()
button_ok.pack()

window.mainloop()