We propose a carbon-based logic gate where AND, OR, and XOR logic operations are available. It consists of a carbon nanotube (CNT) ring coupled to three external CNT-leads, two of them used as input channels, and the third as the output channel. The total transmission for two inputs displays characteristic interference effects. The constructive interference leads to total transmission significantly larger than that for a single input and validates AND gates, whereas the destructive interference corresponds to XOR gates. For OR gates a negligible interference between the transmittance of each single inputs is needed.