The transistor Q3 in your project seems to be serving as a switch or an amplifier in the circuit. It's a general-purpose NPN transistor (2N3904TFR by ON Semiconductor) and, as such, can be used for a variety of applications.

The specific functionality is determined by how it is connected in the circuit. To make an educated assumption, here is how Q3 is connected according to the net connections:

From the connections, one can infer that Q3 may be acting to control the current through LED HIJAU, with the base current possibly being influenced by LED MERAH's state. The exact operation would depend on the full design, including how power and ground connections are made, as well as other components not directly linked to Q3.

Please confirm if this aligns with your design intentions, or feel free to provide additional details if a more specific interpretation is required.