Background: The establishment of health insurance marketplaces (HIMs) offering a market-based response to the need for health care reform was one of the primary provisions of the Affordable Care Act (ACA). The HIMs provided a virtual space in which insurers could compete to provide coverage for consumers who lacked traditional methods of access. Enrollment is crucial to the sustainability of HIMs; however, increasing numbers of insurers are withdrawing from the market, leaving concentrated power to drive up premium prices. Questions: This paper examines the various factors of HIM enrollment by answering two questions: (1) What are the external factors that influenced HIM enrollment of the uninsured during the first three years of implementation? (2) What are the internal mechanisms of a HIM that drove enrollment of the uninsured during the third open enrollment period? Answers to these questions are then used to draft policy recommendations to improve HIM enrollment. Methods: A panel study of state level (external) factors from all HIMs were analyzed using a fractional logit model. Key explanatory variables were chosen from program uptake literature. The study regarding internal factors of HIM enrollment required a multilevel model of county and state level data from 43 HIMs. Key explanatory variables were chosen based on their ability to measure the 3Cs, a previously untested framework that suggested the key features of a successful HIM. Results: External drivers of HIM enrollment of the uninsured were: Medicaid expansion, education and gender of the population, and the average amount of federal subsidies available to state consumers. Medicaid expansion and educational attainment remained significant controls in the second study. In terms of the 3Cs, the number of insurers and generosity of financial aid (competition) increased enrollment. Increased numbers of filter and sort features (commoditization) were shown to have a negative effect on enrollment. Implications and Conclusions: In testing the 3Cs, there is utility in using it to analyze HIM enrollment success; however, suggested modifications may enhance its capacity as a predictive model. State level variables and measures of the 3Cs, external and internal factors of HIM enrollment, suggested common problems with health literacy and affordability of plans. Policy recommendations that follow from these observations can ameliorate the problems.