1. First, determine the size of the stairs according to the space they occupy. Generally speaking, the height of each staircase should not exceed 20 cm, and the depth of each step should not be less than 30 cm.
2. Set stairs on the 3m * 3m platform. The common practice is to set the stairs in a corner and use two adjacent walls as the side of the stairs.
3. Choose the shape of the stairs according to your actual needs and preferences. The back stairs can be curved or right-angled. In the space of 3 m *3 m, L-shaped, U-shaped or 180 degree bend can be considered.
4. Calculate the number and height of stairs according to the selected shape and size. Ensure that the height and depth of each step meet the ergonomic requirements.
5. Add handrails and railings on the walls of stairs to provide better safety.
It should be noted that the above is only one possible way to set up the back stairs and only provides basic considerations. The actual design needs detailed planning and measurement according to specific conditions and requirements. If you need a specific stair design scheme, it is recommended to consult a professional decoration designer or architect, who can provide you with a more accurate and reasonable design scheme according to your needs and space conditions.