3D cell culture in the extracellular matrix (ECM), which not only provides structural support to cellular constituents, but also initiates regulatory biochemical cues for a variety of important cell functions in tissue, has become more and more important in understanding cancer pathology and drug testing. Although the ECM-gel has been used in cell culture both in bulk and on-chip, previous studies focused on collective cell behavior rather than single-cell heterogeneity. To track the behavior of each individual cell, we have developed a gel-island chip, which can form thousands of islands containing single cells encapsulated by the desired ECM. Optimized by Poisson's distribution, the device can attain 34% single cell capture efficiency of the exact number of single cells per island. A good culture media exchange rate and high cell viability can be achieved in the gel-islands. The cells in the islands can be automatically counted for high-throughput analysis. As a proof of concept, we monitored the proliferation and differentiation of single Notch+ (stem-like) T47D breast cancer cells. The 3D collagen gel environment was found to be favorable for the stem-like phenotype through better self-renewal and de-differentiation (Notch- to Notch+ transition). More interestingly, we found that the Notch- de-differentiated cells were more resistant to doxorubicin and cisplatin than the Notch+ cells. Combining the 3D ECM culture and single cell resolution, the presented platform can automatically analyze the individual cell behaviors of hundreds of cells using a small amount of drug and reagents.