You could just put the middle thing directly at world center, move one object along either the x or z axes (not both), then duplicate, go to the attributes and add a negative if you moved it in the positive direction, or remove the negative if you moved it in the negative direction, and boom, it's directly on the other side, with everything equidistant. No guesswork and no math. Or did I miss something and there's a reason everyone is trying to be so complicated about it? Anyway, then you can group the three objects and position wherever you like and they stay together.