Recently, indium tin oxide has been very attractive for thermo-optic (TO)-controlled silicon photonic devices, due to its high transparency, strong TO phase dependency, high conductivity, and CMOS compatibility. These properties enable the reduction of the gap between the silicon core and the heater, leading to a considerably low electric power consumption and high switching speed. We present an investigation of a numerical simulation to design and optimize a compact ITO microheater for tuning the TO phase shift in a proof-of-concept of a three-mode converter (TMC) with broadband and ultralow loss properties. We show that the TMC can operate the 3-dB bandwidth up to 100 nm. In addition, the designed device can attain relatively large fabrication tolerances for width and height of ±50 and ±5  nm, respectively. In addition, the proposed mode converter consumes a total power of less than 90 mW and obtains a fast switching time of less than 8  μs. Moreover, the device can be integrated into an estimated compact footprint of 8  μm  ×  2160  μm. Such excellent performances make ITO a strong candidate for low-loss TO phase shifters and open up an alternative method for realizing ultrafast and high-speed mode division multiplexing systems and large-scale applications.