At stumps, Assam were reduced to 30 for one in their second innings with Pallavkumar Das batting on 21.
Resuming on 77 for no loss, Karnataka declared their second essay on 394 for eight today with Samarth being the top scorer with 131 off 199 balls, including 12 boundaries.
Mayank Agarwal (47), CM Gautam (44), Shrayas Gopal (41) and Shishir Bhavane (40) also chipped in with crucial runs.
Assam require 358 more runs on the fourth and final day tomorrow with nine wickets remaining.
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Brief scores:
Karnataka: 187 & 394 for eight decl. (Ravikumar Samarth 131, Mayank Agarwal 47; Swarupam Purkayastha 3/78).
Assam: 194 & 30 for one in 11.1 overs (Pallavkumar Das batting 21).