The first fitting takes place when the suit is still in its basted form. The tailor checks the shoulder line, chest and body length on your own figure.
The second fitting focuses on the sleeves, lapels and waist suppression. This is when small differences between the shoulders or sleeve lengths are resolved.
The final fitting checks the finished garment as a whole: how it falls when you stand, sit and move. A well-fitted suit must feel comfortable in all three.