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Hello. My name is Alex.Thank you for your question.I will be happy to provide you with information you are seeking for educational purposes only.
Unfortunately, an owner of a 3 unit building would only be required to install railings if such railings are required by the local code based on the height of the staircase.
However, if such railings are no required by the code and the building has passed inspections and has a valid certificate of occupancy, then such railing would have to be installed by the tenant at the tenant's expense and would have to be removed once the tenant moves out.
I wish you the best of luck!