This is what I got from a friend in the US.
The three-pilot scenario:
1. The instrument student logs PIC because he is the sole manipulator of the controls of an aircraft he is rated for. 61.51(e)(1)(I)
2. The pilot in the right seat is the acting as PIC and performing safety pilot duties under 91.109(b)*. He logs PIC as he is PIC in a flight operation that requires more than one pilot. 61.51(e)(1)(iii).
3. The CFI-I who gives instruction logs PIC 61.51(e)(3).
Each person in the airplane has a rule that sepcifically alows him to log the time.
