In recent years, perovskite solar cells (PSCs) have attracted considerable attention due to their low-cost and simple fabrication. Among the perovskite materials, Cs/FA/MA triple cation perovskites, because of its outstanding photovoltaic (PV) properties and stability, have been developed. In this research, triple cation PSCs were fabricated by using a sequential deposition method (SDM) without anti-solvent in ambient air. A novel low-cost mixed solution of ethanol/MACl (Et-M) was introduced into the FAI/MABr precursor to boost crystal grain size and surface quality of triple cation perovskite films. Photovoltaic devices based on Et-M additive demonstrate a higher averaging efficiency of 13.18% than 9.49% of the without additive devices. Results revealed that our new suggested Et-M additive increase crystal grain size more than the conventional MACl additive method in ambient air. Additionally, the defects passivation by Et-M additive increased the ambient and thermal stability of devices. It led to Et-M based devices retain 84% of their initial efficiency (76% for devices with MACl additive) within the 1500 h aging time in dry airbox (30% RH). Obtained results show a low-cost and simple method to improve PV performance and stability of fully ambient air processed triple cation PSCs.