Shi Tingmao's fourth dive in the 2016 Rio Olympic 3m final was "Forward 3½ Somersaults" with a difficulty coefficient of 3.1.
How are such values determined?
In general, is the difficulty coefficient fixed or variable over time?
The values come from the FINA Table of Degrees of Difficulty*
The tables are a matrix that combine the direction and number of any spins and twists, with the shape that is held during the dive.
As you can see, a Forward 3½ Somersault combined with a Pike results in a 3.1 difficulty coefficient.
* The link provided is for the 1m and 3m springboards. For 5m, 7.5m and 10m platform diving see here