from tkinter import *
from tkinter.filedialog import *

def multi_foo(string):
    screen.insert(END, string)

w = Tk()
screen = Entry()
b1 = Button(text='A',command=lambda event: multi_foo("Aa"))
b2 = Button(text='B',command=lambda event: multi_foo("Bb"))
b3 = Button(text='C',command=lambda event: multi_foo("Cc"))
screen.pack()
b1.pack()
b2.pack()
b3.pack()
w.mainloop()