You could use python to achieve this. This function will return 2 or 3 random string characters.
import fme
import fmeobjects
import random
import string
def random_letters(feature):
    length = random.choice(Â2,3])
    feature.setAttribute('letters', ''.join(random.choice(string.ascii_uppercase) for _ in range(length)))
    pass
random_letters.fmw
@madwarren's suggestion is probably the easiest way, but if you want/need to avoid python, you can use a random number generator (1-26) followed by an attributeValueMapper to map the 26 numbers to A-Z.
You can also use a random number generator, with a min value of 97 and a max value of 122 followed by a a charactercodereplacer.
Â
Or just the charactercodereplacer with @rand