Here, the authors predict that topological Josephson trijunctions generically display a nonzero quantized transconductance. This result originates from a combination of two different aspects of topology: topological superconductivity in the leads, thus hosting Majorana end modes, and a synthetic topological band structure in the multiterminal Josephson junction. Many groups are currently studying multiterminal junctions using various superconducting hybrid platforms. An experimental check of this prediction would constitute an important milestone on the roadmap toward demonstrating Majorana braiding in trijunctions.