How can I make a real garden on a 3rd floor balcony?

Try planting multiple items in each pot. Add additional, different-sized containers, hanging pots or window boxes (attached to your balcony railing) to maximize your growing space.

do a quick search on Pinterest, there are a number of ways to do it using various materials (pvc pipe, eavestrough) that would be safe for what you are wanting to do

Things to consider: What is your light exposure, what is your growing zone? Are you thinking of just decorative or also edible? Is the balcony well constructed or a little wobbly? How will you be watering these plants and where will runoff go? Will you be bringing them inside during the winter? Absolutely need to use appropriate container soil not garden soil or "dirt".