Introduction To minimize trauma and cost of treatment, oral health practitioners have successfully rehabilitated full arches by supporting the prostheses on four implants. However, there is no consensus whether less than four implants supporting full mandibular arches would provide similar clinical outcomes to other well-established all-on-four alternative. Objective To identify, summarize, appraise, and compare the clinical outcomes evidence of three-implant fixed full-arch prostheses in completely edentulous mandibular patients. Materials and methods This overview of systematic reviews (OoSRs) will include secondary synthesis studies (i.e., systematic reviews with or without a meta-analysis). A three-step search strategy will be conducted in MEDLINE (Ovid), EMBASE (Ovid), Cochrane Database of Systematic Reviews, Scopus, Web of Science (WoS Core Collection), and Google Scholar. Grey literature and a manual search in 12 specialized journals will also be conducted. Three independent reviewers will screen all retrieved articles for eligibility, extract data and assess the methodological quality of the included studies. The results will be presented as tables or narrative synthesis. The studies will be evaluated for risk of bias by ROBIS and methodology quality by the AMSTAR-2 tool. If new primary studies are identified, a meta-analysis will be conducted. Certainty of the evidence will be assessed to answer the following focused research question: In edentulous mandibular patients, what are the implant and prostheses clinical outcomes of three-implant fixed full-arch prostheses compared to other all-on-x solutions? Discussion There are some systematic reviews about the use of fixed complete dental prostheses supported by three implants; however, their clinical outcomes related to the other all-on-four plus solutions are conflicting. So, an overview on this topic is required to provide recommendations. Review registration number International prospective register of systematic reviews (PROSPERO) ID#: CRD42021262175. National Institute for Health Research (NIHR) and Centre for Reviews and Dissemination, University of York, York, United Kingdom.