@WeatherVane is correct. You've been presented with three options, one of which should be selected before operating the equipment. Another way of expressing the same sentence is:
Before operating the machine do one of the following: (1) attach the platform entry chain, (2) lower the platform mid-rail, or (3) close the entry gate.
Having written technical documentation in the past, it's a bit surprising to see the phrase "before operating" at the end of the sentence as it's the "test" or point during the procedure where one must pause to evaluate something else. In good technical writing, that usually comes first. However, not knowing the details of where this came from, it's impossible to judge. If the writers only had a warning sticker worth of space to work with, brevity may have been required.