Background: Previous studies have evaluated the initial stability of uncemented tibial components in revision total knee replacement (rTKR) in the presence of an Anderson Orthopaedic Research Institute (AORI) Type II tibial defect. This study sought to evaluate similar metrics in the severe Type III (AORI TIII) defects with combined uncemented stem and sleeve fixation, specifically, the effect of varying the stem's length and tibial canal engagement upon stability and bone strain. Method: Finite element models generated from the CT scans of 4 tibias with Type III defects were used to investigate the primary stability, in terms of the bone–implant composite peak micromotion (CPM) and microstrains (CPS), achieved after virtual implantations with and without stems. Results: A stemless rTKR had increased metaphyseal CPM and CPS compared to all stemmed implants. Significant area of the bone supporting the stemless rTKR had CPS greater than bone yield (7000 με). Short engaging stems (≤150 mm construct length), could not achieve reliable engagement in the diaphysis (canal fill ≤ 50%), leading to insufficient reduction of CPS (≥5000 με). Longer engaging stems (170–220 mm construct length), were able to reliably engage the diaphysis (fill ratio ≥ 75%) resulting in CPS ≤ 5000 με. Although, non-engaging stems resulted in increased CPM and CPS compared to engaging stems, long non-engaging stems (170–220 mm construct) appeared to provide additional stability to the rTKR compared to stemless rTKR. Conclusion: The results indicate a likely correlation between uncemented stem engagement and metaphyseal CPS in Type III defects. Excessive strain within the supporting metaphyseal bone is likely to lead to rTKR migration and loosening.