Breast cancer is one of the cancers with the highest incidence and mortality rates in women in Turkey as well as in the world. Tumor micro environment comprises of cancer and normal cells, extracellular matrix, soluble biological and chemical factors. Research has shown that cell shape, adhesion, migration, response to growth factors and drugs are different in 2D and 3D culture. Today, only 8 out of 100 anti-cancer clinical trial gives effective results. 3D cell culture systems have shown to be a necessary step between in vitro, in vivo and clinical studies. Therefore, it is necessary to better understand the interactions of cancer cells with their micro environment, for which new cell culture setups are required. The most apparent disadvantage of widely used 3D cell culture setups is the lack of stromal cells. The systems to be developed should both provide a 3D environment and comprise multiple cell types. The drug screen in 3D tri-culture method with a lab-on-a-chip device, that will be developed in this study will be able to answer these needs. Cell lines that represent different breast cancer types alone or together with stromal cells were cultured in 3D in the to be developed lab-on-a-chip; by determining the effects of drugs with different targets on the viability and distribution of cells, a drug screening method is developed.