Add new keys to a dictionary while incrementing existing values

I am processing a CSV file and counting the unique values of column 4. So far I have coded this three ways. One uses "if key in dictionary", the second traps the KeyError and the third uses "DefaultDictionary". For example (where x[3] is the value from the file and "a" is a dictionary):

First way:

if x[3] in a:
    a[x[3]] += 1
else:
    a[x[3]] = 1

Second way:

try:
    b[x[3]] += 1
except KeyError:
    b[x[3]] = 1

Third way:

from collections import defaultdict
c = defaultdict(int)
c[x[3]] += 1

My question is: which way is more efficient... cleaner... better... etc. Or is there a better way. Both ways work and give the same answer, but I thought I would tap the hive mind as a learning case.

Thanks -