Place an empty gearbox. Right click one side with monkey wrench (this will put the inner axle through it) and then click on that same side with normal wrench (this will put a gear on that side) and then put another gear on the side next to the first gear (so that they are interlocked). Now you have a 3-way gearbox that connects those 2 oposite sides together with an axle. Try it so that you understand how it works.
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