Background: Childhood illnesses, including fever, acute respiratory infections (ARI), and diarrhea, remain leading causes of morbidity and mortality among children under five in low- and middle-income countries like Bangladesh. These conditions are influenced by socioeconomic, environmental, and behavioral factors, necessitating a comprehensive understanding of their prevalence and determinants to inform public health strategies. Objective: This study aims to determine the prevalence and associated demographic, socioeconomic, and environmental factors for fever, ARIs, and diarrhea among children under five in Bangladesh, thereby providing insights for targeted interventions. Methodology: Utilizing data from the Bangladesh Demographic and Health Survey (BDHS) 2017–18, this cross-sectional study examined a weighted sample of 8,421 children under five. Variables including demographics, socioeconomic status, and health-related factors were analyzed. Prevalence of fever, ARI, and diarrhea was recorded, with logistic regression models estimating odds ratios to identify associations between childhood illnesses and selected predictors. Results: Among children aged 0–59 months, boys exhibited a higher prevalence of fever and ARI compared to girls. Children younger than 12 months showed the highest rates of all three illnesses, particularly ARI. Breastfeeding status, maternal age, and socioeconomic factors (e.g., parental education, wealth index) significantly influenced illness prevalence. Additionally, safe water access and sanitation were linked to lower fever and ARI prevalence. Vaccination status was also associated with illness patterns, highlighting complex health disparities. Conclusion: The study identifies critical factors affecting the prevalence of fever, ARI, and diarrhea in children, emphasizing the need for interventions addressing environmental and socioeconomic risks. Improved water, sanitation, vaccination coverage, and healthcare access are recommended to reduce disease burden among children in Bangladesh.