Someone

import string

LETTRES = string.ascii_letters
MINUSCULES = LETTRES[:26]
MAJUSCULES = LETTRES[26:]

t = open('3.txt','r')
work = t.readlines()

tab = ""
y = 1
verif = 0

LETTRES = string.ascii_letters

for element in work:
    x = 0
    y = 0
    verif = 0
    while ( x < len(element) ):
        if element[x] in MINUSCULES:
            while ( y <= 3 ):
                if element[x+y] in MAJUSCULES and element[x-y] in MAJUSCULES:
                    verif = verif + 1
                if verif == 3:
                    tab = tab + element[x]
                y = y + 1
        x = x + 1

print tab