Join multiple dictionary and flatten joined list in Python2

import random
import csv

def flatten(L):
    if isinstance(L, list):
        return reduce(lambda a,b: a + flatten(b), L, [])
    else:
        return [L]

def main():
    dict1 = {}
    for i in range(0,100,2):
        dict1[i] = [random.random(),random.random()]
    dict2 = {}
    for i in range(0,100,3):
        dict2[i] = [random.random(),random.random()]
    dict3 = {}
    for i in range(0,100,5):
        dict3[i] = [random.random(),random.random()]

    f = open("./test.csv", "a")
    writer = csv.writer(f)

    for k1,v1 in dict1.items():
    if k1 not in dict2:
            continue
    if k1 not in dict3:
            continue
    v2 = dict2[k1]
    v3 = dict3[k1]

    l = []
    l.append(k1)
    l.append(v1)
    l.append(v2)
    l.append(v3)
    writer.writerow(flatten(l))

if __name__ == "__main__":
    main()