The paper presents a novel study on film cooling effectiveness of a 3D flat plate with a multihole arrangement of mixed hole shapes. The film cooling arrangement consists of two rows of coolant holes, organized in a staggered pattern with an L/D (length to diameter ratio) of 10. The two rows consist of varied combinations of triangular and semi-elliptic shaped holes for the enhancement of film-cooling effectiveness. The results were obtained for a coolant to mainstream temperature ratio of 0.5 and a blowing ratio of 1.0. The computed flow temperature fields are presented in addition to the local two-dimensional streamwise and spanwise distribution of film cooling effectiveness. Validation of the results obtained from the turbulence model has been done with the experimental data of centerline film cooling effectiveness downstream of the cooling holes available in the open literature. The results showed the rapid merging of coolant jets emerging from front row of multiholes with the secondary staggered row of mixed holes. Due to the mainstream–coolant jet interaction, the strength of the counter rotating vortex pair was mitigated in the downstream region for certain arrangement of mixed hole shapes. The optimal hole combination with maximum overall effectiveness has been deduced from this study. The best configuration (M.R. VI) not only favored for the developed film, but also enhanced the averaged film cooling effectiveness to a large extent.