There are a few other ways of doing the XOR. All of them have 1 or more hidden units making the minimum units needed 4. Look at the units for AND, NOT and OR in the textbook and make sure you understand what XOR does.
After I designed mine I wasn't sure and looked around a little only to find my structure 100% but my bias weights on two nodes completely out! Rethinking how XOR worked made it all clear(was a very silly mistake) and I think it's correct now.