It was Erapalli Anantharao Srinivas Prasanna. He was a spin bowler, specializing in off spin. Prasanna played his debut Test cricket match at Madras against England in 1961. He retired in 1978, after a tour of Pakistan which also signalled the exit of Bishen Bedi and Bhagwat Chandrasekhar from the national team.
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The third one you can choose between S.Venkatraghavan and Erappali Prassanna . both of them are great bowlers and off spinner of repute. Pataudi and Wadekar choose the third spinner according to pitches and the merits and hence all i can say is S.Venkatraghavan and E.A.S. Prassana filled the third spinner during 70s.