Challenges in improving catalysts for electrochemical CO2 reduction require a clear understanding of the reaction mechanisms that lead to products of higher value. In this work, we use density functional theory (DFT) to determine the most competitive coupling mechanisms leading to C3 products on Cu(100) and Cu(511). We exhaustively consider surface coupling pathways between CO* and different C2 intermediates. On Cu(100), CO* coupling with acetaldehyde was identified as a notable step for C3product formation. On Cu(511), local field stabilizations enable an additional coupling step between HCCH* and CO*. This suggests that there is more than one possible pathway toward forming C3 intermediates. Our simulations show that much like C2formation, C3 formation prefers stepped surfaces with (100)-like sites and that local field stabilization can play a pivotal role in certain coupling steps.