Introduction: In Brazil, an estimated 73,610 new cases of breast cancer were expected annually among women during the 2023–2025 triennium, corresponding to a risk of 66.5 per 100,000 women, with higher incidence in the Southeast and South regions and lower rates in the North. In the same period, 17,010 new cases of cervical cancer (15.38 per 100,000) and 7,310 cases of ovarian cancer (6.62 per 100,000) are expected annually. Despite advances in screening and cancer control, population-based net survival estimates remain scarce in Brazil due to limited data availability and lack of standardized analytical methods, restricting comparisons of cancer outcomes across regions. Survival estimates are essential to understand regional disparities and the clinical and sociodemographic determinants of cancer prognosis. Objective: To estimate and compare overall and net survival rates for breast, cervical, and ovarian cancers in Rio Grande do Sul, Brazil, using population-based data, including age-standardized estimates. Methods: The Agatha study is a population-based retrospective cohort including 1,785 patients with breast cancer, 369 with cervical cancer, and 184 with ovarian cancer, aged 15–99 years, diagnosed between 2016 and 2018. Data were obtained from the Population-Based Cancer Registry of Porto Alegre, Rio Grande do Sul, Brazil. Passive follow-up was conducted through probabilistic record linkage with the Mortality Information System. Overall survival (OS) was estimated using the Kaplan–Meier method, and net survival was estimated using the Pohar–Perme estimator. Age standardization was performed using the Miranda and Corazziari methods. Results: For breast cancer, the 5-year overall survival was 82.38% (95% CI: 79.49–85.27). Age-standardized OS was 83.42% (95% CI: 81.65–85.19) using the Miranda method and 80.64% (95% CI: 78.63–82.65) using the Corazziari method. Net survival was high, with a crude estimate of 89.7% (95% CI: 87.8–91.7), remaining stable after standardization. For cervical cancer, crude 5-year OS was 57.7% (95% CI: 51.4–64.7). Age-standardized OS ranged from 48.14% to 52.7%, depending on the method. Net survival was 60.1% (95% CI: 53.6–67.4), decreasing after standardization to approximately 53–56%. For ovarian cancer, crude 5-year OS was 36.5% (95% CI: 29.5–45.1). Age-standardized OS increased substantially, reaching 51.13% with the Miranda method and 47.26% with the Corazziari method. Net survival rose from a crude estimate of 39.4% to over 52% after age standardization. Conclusion: This study provides standardized overall and net survival estimates based on robust population-based data, addressing an important gap in Brazilian cancer surveillance. The use of complementary age-standardization methods improves comparability across populations, supporting national and international monitoring of cancer outcomes. The integration of cancer registry teams, mortality data systems, and academic partners strengthened analytical capacity and promoted methodological transfer. Limitations include the lack of TNM staging and other clinical variables, which are relevant for prognostic assessment and survival interpretation.