Neuromorphic systems are essential for next-generation neuro-robotics and neuro-prosthetics with biological signal processing and memory functions. In this work, we propose a 3D integrated neuromorphic system for chemical sensing with 3D printed components, including a sensor, an electrical oscillator, and a synaptic transistor, which mimic sensory receptors, neurons, and synapses in biological systems, respectively. The 3D neuromorphic system was fabricated by 3D printing of multiple materials, resulting in significant improvements in the volumetric efficiency of the system. Through this system, different ion concentrations are detected, self-activated, and transmitted. Finally, the electrochemical neuromorphic sensing function is demonstrated by the integrated 3D neuromorphic system with artificial sensory receptors, artificial neurons, and artificial synapses for application in the detection of low concentrations of soil nutrients in smart farms.