Place two blocks next to each other (distance: one block). Install two redstone-lanes (both connected to a lever) which end in these blocks.
Put two redstone-torches at the other side of these blocks and extract the signal of the torches by connecting the redstone-lanes.
Build another block right next to the extracted signal, and put a redstone torch at its back, too.
Put the output-lane behind this torch.
The output becomes positive, if all inputs are positive.