Herein, the possibilities of tuning oscillation frequency and power in terahertz (THz) graphene nanoribbon (GNR)‐based avalanche transit time (ATT) sources by introducing a third terminal on their planar structure are discussed. In this regard, a three‐terminal planner impact ATT (IMPATT) structure based on GNR on oxide (SiO2) is proposed. Inherent power combining capabilities of parallel‐connected IMPATT structures are utilized to increase the power output at THz frequencies. Static, high frequency, and noise simulations are carried out using in‐house simulation codes based on self‐consistent quantum drift–diffusion model. Results show that around 0.7–5.3% and 9.0–13.3% modulation of frequency and power output are achievable, respectively, in the GNR IMPATT oscillators operating in the frequency range of 1.0–10.0 THz, respectively, by applying a suitable amount of voltage at the third or controlling terminal; however, the enhancement of noise level at the output is observed to be insignificant as a result of using the active third‐terminal control.