The characters 她 and 它 exist in old Chinese, but they were not third person pronouns and were not pronounced "ta". 她 used to be the same as 姐，meaning mother, while 它 was the original form of 蛇（snake）.
Using 她 and 它 as pronouns is indeed a rather recent development, mainly by the cohort of May Fourth Movement as a way to mordernise Chinese and make translation of English works easier. The usage of 她 as female third person pronoun was suggested in 1917-1918 by poet Liu Bannong(刘半农). Around the same time, 伊 (yi, female pronoun in the Wu dialect) was also commonly used as female third person prnoun, but gradually fell out of favour. I'm not exactly sure when and how did the 它 begin as a pronoun, some sources suggest it's the same as 她。
他 assumed its current form during Libian (隶变) 2nd Century BC, its original form was 佗(burden). It probably was populoarised as a pronoun during Ming dynasty (14th century).