Which one to use Part 3 or Part 9 ?

Here's the guidline for that: Part 9 applies to Group C/D/E/F(2&3) buildings, given that the building is less than or equal to 3 storeys AND less than or equal to 600 m² in building area (footprint). If either threshold is crossed it triggers Part 3. Group A, B, and F(Div.1) are always Part 3 regardless of size. Referebce OBC: 9.1.1.2., Div A 1.1.3.2.