Background: Knee osteoarthritis (OA) is a prevalent condition among adults that leads to knee joint pain and dysfunction. Over the past two decades, local intra-articular knee injection therapy has gained popularity due to the advent of platelet-rich plasma (PRP), hyaluronic acid (HA), and the novel peripheral blood-derived mononuclear cells (PBMNCs). This study aimed to compare the therapeutic efficacy of intra-articular injections of PBMNCs, HA, and PRP combined with hyaluronic acid (PRP-HA) for treating degenerative knee OA classified as stages II and III, according to the Kellgren and Lawrence (KL) scale. Methods: This retrospective observational study involved adults with moderate-to-moderately severe knee OA treated at the University Hospital of Catania, Italy. The subjects were divided into three groups and treated with intra-articular injections of HA, PRP-HA, or PBMNCs. The outcome measures assessed were as follows: the Numerical Rating Scale, the Western Ontario and McMaster Universities Arthritis Index, the Timed Up and Go, the International Knee Documentation Committee score, a 10-meter walking test, and the Physical score and the Mental score on the SF-12. This study included a total of 46 adults, 30 females and 16 males, with a mean age of 63.7 ± 10.9 years. Results: HA, PRP-HA, and PBMNCs demonstrated comparable effectiveness for improving the NRS score and all the other outcomes at 6 months. Additionally, PRP-HA and PBMNCs also enhanced knee flexion and the International Knee Documentation Committee score. However, none of the three treatments led to a significant improvement on the 10-meter walking test. No serious adverse effects were reported. Conclusions: In this study, injections of HA, PRP-HA, and PBMNCs all demonstrated positive outcomes for up to 6 months post-treatment in the subjects suffering from knee OA.