'X' is among the world's oldest civilisations. It was first identified in 1921 at 'Y' and then in 1922 at 'Z'. 'X' is known to have consisted of two large cities that turned out to be 'Y' and 'Z' and more than 100 towns and villages, often of relatively small size. The civilization was literate, and its script, with some 250 to 500 characters, has been partly and tentatively deciphered with the language has been indefinitely identified as Dravidian. Identify 'X', 'Y' and 'Z' respectively.