from tkinter import *
from tkinter.filedialog import *
def a_foo():
    screen.insert(END,'Aa')
def b_foo():
    screen.insert(END,'Bb')
def c_foo():
    screen.insert(END,'Cc')

w = Tk()
screen = Entry()
b1 = Button(text='A',command=a_foo)
b2 = Button(text='B',command=b_foo)
b3 = Button(text='C',command=c_foo)
screen.pack()
b1.pack()
b2.pack()
b3.pack()
w.mainloop()