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