Research other airbnb listings in your area and see what the pricing and amenities are. Consider what you want to offer your guests and how much money. Bottled water and cookies on arrival? I nice coffee maker?
Research insurance and tax issues in your town/county/province.
Consider who is available to speedily clean the unit between back-to-back bookings. How will you get keys to the guest at all times of day/night. (Some hosts limit times that guests can check-in, but travel plans don’t go as predicted).
Read as many postings on this forum as you can - but don’t let them scare you off, just inform you. There are a lot of good postings about bedding, beds, keys, etc.
Think of what you should invest in - new bedding? Extra sheets? Towels? Keys? Shampoos? Soaps?
It sounds like the 3rd floor is a better apt but the 2nd floor is easier to access, so they both have a selling point.
Let us know how it goes!!