High intensity focused ultrasound (HIFU) is a non-invasive thermal ablation technique. To perform the ablation procedure safely, temperature monitoring is employed to preserve healthy tissues while simultaneously ensuring that the targeted region is completely ablated. Ultrasound (US) thermometry techniques have the advantages of cost-effectiveness and portability over other medical imaging modalities such as MRI. We propose a 3D US thermal monitoring method for HIFU ablation. A US element and sampling device are used to acquire time-of-flight (TOF) information, from which we reconstruct speed of sound (SOS) images to detect the temperature increase during the ablation. We use a physics-based HIFU simulation to segment the ablated region of interest (ROI) to cope with the sparsity of the recorded data. HIFU thermal ablations were performed under MR monitoring on a phantom and the results from the proposed method were compared with MR thermometry. On average, the difference between those two datasets was 1.3°C in the ROI around the ablation focal point, which verifies the feasibility of the proposed method.